I'd go with two sources: GNC and Vitamin Shoppe.
GNC is one of (of not the) easiest to find and you can get the Gold Card. The best GNC stores are the ones on military bases. Their prices are usually 10-15% lower than the other GNC stores and you can use the Gold Card all the time, not just the first 7-10 days on the month. And, at all the GNCs, you can use the Gold Card on marked-down products, as well.
Vitamin Shoppe is a good store, but I'm finding that it may be even better for those who order online. Too many online shops claim to have great deals. But, the money you're supposedly saving get negated by ungodly shipping costs. Vitamin Shoppe has standard shipping costs of $5 that gets your stuff to your door in 4-6 business days. As of late, VS has "Super Saver Shipping", which is $3 and delivers in 3-10 business days and is FREE for all orders of at least $100 (that includes FPO/APO addresses for all you military folks). There are also coupons with promotions of "20% off $100". Simply put, you get 20% off your purchase, if you buy at least $100 worth of stuff.
