SurLaTable.com and PastryPortal.com sell mini pie shells, otherwise called tart shells or mini tart shells. Clearbrook Farms, which manufactures these mini pie shells, also sells them at ClearbrookFarms.com.
When premade mini pie shells are unavailable, one option is to make them out of standard frozen pie shells. For example, Pillsbury's recipe for lemon mini tarts involves flattening a frozen pie shell into a 15-inch sheet with a rolling pin, and using a circular cookie cutter to cut out the mini pie shells. One Pillsbury frozen pie shell contains enough dough to make 16 3-inch miniature pie shells, according to Pillsbury.com.