bodybuilding.com is the place for all your needs. Supplements are cheap compared to GNC and such. Just bought some stuff today from bodybuilding.com and I'm going to try out their "tornado" cup.
GNC is a retail store (no matter how many times they may advertise that they're not, yes, they're a retail store). What does this mean for you and me? It means that they're going to mark up their products as much as 100% of the store's purchase price. You're better off going to a discount or warehouse-type store for your supplement needs.
big fan of T-nation.com, as long as you go in expecting them to push their supplements. I've used some of their supplements as well and I have had good results.