Gee...in your area. Well, as far as thrift stores go, Goodwill seems to be everywhere, and so might be the American Veterans (AmVets) thrift stores. The ones in my area are called the Village Discount Outlets. Local consignment shops vary incredibly, so I can't give you any leads there other than tell you I'm sure they're there.
Look in the phone book under clothing and see what you find. Different phone books categorize things different ways, so you may need to try several terms/words before you hit on exactly what you're looking for. If you don't want to make clothes yourself and are having a hard time with ready-made options, you might try looking up a sewer. (As in a person who sews, not the place where the filthy water goes.) NYC is a big place--they may even have a sewing school whose students might find making clothes for you to be a fun challenge. Schools will often discount the prices significantly since you are accepting the possibility of imperfections.
Or you could always stop the hippies you see out in public and beg them to tell you where they got their clothes.
Get involved with environmental causes, meet some hippies, and follow them around as they obtain clothing.
Q: How many poets does it take to change a light bulb? A: 1001...one to change the bulb, 1000 to say it's already been done.