Where to buy Basil???
Yes, assuming Pittsburgh area, the strip is the place. Guess I'm also assuming you're talking about dried basil and not fresh.
Specifically, you could try Pennsylvania Macaroni which sells bulk herbs/spices right inside their retail entrance. They also have a large wholesale operation. You may just want to give them a call and let them know how much you need. http://www.pennmac.com/
I'd prefer you support local businesses such as Penn Mac, but sometimes for one-off items like this, Restaurant Depot is really handy. They carry all kinds of food items and equipment, including large containers of herbs/spices/etc. It's not open to the public, you'll need a business tax id to become a member (free).
If you're really talking large quantities, I suspect you may want to find a supplier online who can deal with that kind of volume.