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.
