A local bar that serves Belgian beers out of the bottle would be your easiest place to rack up those bottles in a hurry.
Of course, collecting them yourself is the most enjoyable way to build up a collection. You need a lot fewer bottles than you might think. Five 750ml bottles holds a gallon of beer, so you only need around 25 to cork a five gallon batch. Quite a few domestic saison and abbey-style beers are sold in that format and can be found in the $6-10 price point. Host a few Belgian beer tastings with friends bringing bottles along could get you to 25 bottles quickly.