www.whitearmor.net <-- Official site for stormtroopers (a.k.a. TKs) in the 501st legion There is a lot of infomation and good tutorials for beginners. rspropmasters is a bit expensive but the quality is pretty top notch. I would recommend them. Other manufacturers are ATA, TM and more. And please stay away from ebay if you are looking for high quality.
Research is the most important thing when it comes to getting a stormtrooper outfit!