PA is one of those states where regulations mean that you can only buy beer either in take-out form from bars (6-packs) or by the caseload from distributors. Presumably, this is meant to curb alcohol consumption, but really I think all it does is force people to drink beer by the case!
Regardless, if loading up on beer is your objective, Springfield Beer Distributors is the choice if you live around West Philly / Center City Philadelphia. Located by the south street bridge, springfield is an old warehouse literally stacked full of cases of beer. They have all national brands, most imports, and many boutique / microbrew brands as well. Especially good for getting microbrews from the mid-atlantic region.
They also deliver, so they are good if you need beer for an event or party.