My suggestion would be to look for a cosplay community and see what they recommend. When it comes to costuming, for example with the 501st Legion, the community will often have sellers. But, be aware that for a high-quality piece, you'll need to spend a lot of money. For example, a good stormtrooper rig will run you at least $500.
Looking over eBay, I'm seeing various sellers with prices ranging from $90 to $100. If you're looking for higher quality, try searching for parts of the costume, rather than the entire costume. The price will be higher, though, if you buy it piecemeal.
Also, another option would be to find patterns and buy the fabric, and pay someone to make the costume for you. This is what most professional cosplayers do, if they don't make the costume themselves.