Where can I find a condenser for an American Standard Allegiance 12 air conditioner?

A condenser what? What part?

Any local supplier should be able to get the condenser for that unit. There are many different brand names that will work for your system. Call around for the best price.

Before you call around, though, get the make and model (and tonnage) of the condenser you're replacing, so that they're sure to get you the right one.