For the majority of my weight loss wardrobe, I'm hunting estate sales and thirft stores. I have a limited wardrobe in anything 18-14, so as I reach each new size, I'll have to buy pants. I can take in the skirts I have and the shirts can be worn loose for a while longer. As soon as spring sales hit the department stores, I'll be wading through their 70% racks, too.
My advice:
Pants are always the first thing to show a size dropped. Invest in one pair of well fitting, attractive jeans every time you drop a size. Keep them in good condition and each size you drop, take it to either a consignment shop, donate to resale or sell it on eBay.
Skirts can usually be taken in once or twice depending on fabric and style.
Shirts can be worn loose or belted, or even under a cardigan or jacket.
Dresses don't have to be skin tight, but make sure the bustline and the waistlines are altered to flatter your new frame.
Once you get down to 5 pounds within your goal weight, that's when you should start looking for more permanent investment pieces - stuff that's near your goal, that can be taken in 1 time to account for any additional loss of inches and that make you feel great when you wear them.