I really don't have enough money to spend on every size of clothing on the way down to my goal weight. Are there any tips or tricks people use to save money until I want to purchase a whole new wardrobe?
A lot of people will suggest browsing through second-hand stores like Goodwill, the Salvation Army, consignment shops, etc. You may have to spend some time digging but you can often find like-new clothes (sometimes still in the original tags!) for extremely cheap. I always carefully look over anything I might pick up at one of those places, checking for undone seams, tears/holes, or stains.
Also look for discount stores that sell the "leftovers" that were swept out of higher-end stores. It depends on the area, but around here we have Susie's Deals, Ross, and Fallas, all good places to find designer clothes for a low price. The last time I was at Susie's Deals they had Lane Bryant and Forever 21+ brand jeans for only $10 and tops as low as $3. When I was at Fallas they had some awesome plus-sized leather jackets for $15 that still had the tags from their original store (showing they'd been $125)!
Good luck in finding stuff! I know it kinda sucks to buy something while knowing it won't fit very long, but hunting for discounts on awesome new clothes can be a lot of fun. And as you look, try to find neutral clothes that can be layered together in different ways so you can get as much use out of them as you can.
I've gone down nearly 70 pounds, and I'm just now starting to need newer things, so you may find you're not having to buy as much as you think, although every one is different. I would suggest (on top of the thrift shop suggestion) that when you do buy, get one good solid piece (like a pair of black slacks) in a size that's just slightly tight. That way you have longer to wear them before they're too big. Anything with spandex in the waist will last longer too, as often you can cut a small hole and "tighten" the elastic with stitches.
I understand that money's tight, but if you can, I'd suggest always trying to have one or two outfits that you really feel good in--even if that just means altering your favorite current outfit when you've lost a size or two.
LOTS can be done with alteration. If you don't sew, do you know anyone who does? Some things are easier to take in than others. Skirts come in a lot easier than pants, especially straight-ish skirts with elastic waists.
BCOutlet.com and OneStopPlus.com are great for bargain shopping. I check both at least once a week, because it seems that the stuff I like goes really quickly.
I also look for drawstring waists a drapey knit fabric, because the more drapey the fabric the more leeway there seems to be in fit.
I also look for clothes that can do double or triple duty. For example I bought a pair of black drawstring knit pants at Target in a very soft, drapey knit. They're so soft they feel like jammie pants, but they actually look very dressy so I'm wearing them as dress slacks right now, but as they wear (and probably fade) I'll downgrade them to casual slacks, and eventually jammies.
I bought a bunch of knit drawstring pants at BCOutlet. I'd bought the brand before, so I knew they ran VERY generously, so even though I'm between a 3X/4X, I bought them in 2X. They were only $2.99 a pair. I used a coupon code so I got free shipping. The red and the teal are nice enough to wear casually, but the blue and green are a bit pastel for my taste, so I wear those to the gym. I bought a bunch of tunic t-shirts for under $6.
The tops and the pants are also in soft knits, so they'll be casual clothes, then gym clothes, then jammies.
I do this with knit dresses too. I buy a soft knit and wear them for going out until they get too big or too worn, then they become jammies.
Oh, and I learned a new trick from my sister - tye-dying. And then learned an even better tip (because it's cheaper) on youtube - tye-dying with bleach. Before deciding that a stained top has to become gym clothes or jammies, I try tye-dying it with bleach first (search on bleach dying on youtube).
I had a very pretty purple graphic t-shirt with rhinestones that had become faded and stained. The rhinestones didn't make it something I'd want to wear to bed, so I decided to tie-dye it with bleach. It came out amazingly adorable and almost new looking. The bleached parts faded to a fuschia pink and the tied parts remained royal purple. I think it actually looked better this way than it did when it was new.
I also bought a solid dark purple 3/4 sleeve cotton button down shirt from Goodwill for like $2 and I just didn't like it, and then I stained it (I hate solid colors for that reason). So I bleach died it and it too came out gorgeous and is now one of my favorite tops. The tied parts stayed dark purple and the bleached parts faded to a really pretty plum.
I have about another dozen tops that I had in my "rag bag" (oh year, after they're done being jammies or if they aren't comfy enough to go to jammies they go to the rag bag in my craft room). I have about a dozen tops that I'm going to try to bleach dye or tie dye to see if I can salvage them. I have about half of them tied.
I have a nice tie dye kit from Michaels that I bought with a 40% off coupon. I bought the biggest kit, because I got a little overexcited about the tie dye process. Now I like how the bleach works so much, I'm not regretting buying it, because even using the tie dye kit it amounts to about a dollar or less per shirt.
Hubby teases me that I'm going to be tie-dying my undies next, and I laugh along (not telling him that I've thought about it).
Ross, Beall's outlet and Marshalls etc are great. Often times have to dig, but its worth it. Also I've had some really good luck at second hand or consignment shops. I also browse ebay, sometimes even find 'lots' of clothes for a really good price. They're not super fancy, but it's something to wear when everything else is falling off. Alterations can be very easy and as little as a couple bucks at a tailor.
I bought a pair of pants at a time, shirts are different for me cuase i can wear them longer then pants. I also use to be a size 20 so i have some of my old clothes.
I have found that I can never find any clothing above around a size 20/22 at Goodwill or thrift stores... and even that big is hard to find.
I have had luck at Ross's... and a lot of their dresses are a knit material... so they tend to fit me in a range of sizes. Before I gained some of this weight back, I had got a couple dresses, and they still fit. I tend to wear a light sweater over them (perfect for this weather right now anyways), because they cling to my tummy a little more than they did when I bought them.
I haven't had much luck buying clothing at other places, but clearance can be a great deal (even stores like Fashion Bug, etc... their online clearance is sometimes crazy cheap, just depends on your style).
Buy at thrift stores or get use to baggy clothing...
Maybe you can tie a shirt in the back with an elastic band so it's not loose, or wear a tight belt on some loose pants...good luck
Clothing
And as you look, try to find neutral clothes that can be layered together in different ways so you can get as much use out of them as you can. 
