Bodybuilding.com is a realiable supplier. Cheap, fast, and good customer service.
However, for protein I love trueprotein.com. Their products are tip-top when it comes to quality. You can customize your stuff any way you want it, but the Team Skip formula will work wonders for most people. Optimum Nutrition 100% Whey isn't bad, but the Team Skip kicks the crap out of it if you're a serious lifter.
1-800-Supplements is a solid vendor and I've also heard good things about Bulk Nutrition although I've never ordered from the latter before.
Stay away from Vitaglo.com. I'd strongly suggest checking reseller ratings before you do any business with them.
I buy a lot of supps and I've been lifting for a long time, so if you need specific advice, feel free to PM me.