Any restaurant food supplier (Sysco, United Foods, etc...) should be able to sell it to you. If you're in a microbrewery rather than a brewpub just have a local restaurant you're friends with order it for you. It usually comes in "ten cans" - 10 lb. cans that come six to a case. I've tried a couple of different brands over the years, but Libby's is by far the best (it makes the best pies too!). You may be able to by the 10 cans at a bulk food store like Sam's. But keep in mind that canned pumpkin is basically just a squash. It has very little flavor of it's own. Most of the flavor people associate with pumpkin pie comes from the spices.
Libby's sells both a spiced & unspiced version of their pumpkin. As you mentioned, unspiced is the way to go.