A 292 would be the best/biggest choice. The pass. side motor mount and fuel pump will swap places, and the 292 has a taller deck, so the exhaust pipe will need some changes. Some of the brackets may need to be changed also. Starter and flywheel might need changing, but its nothing serious if the 292 comes with everything on it.
As to finding one, salvage yards, classifield ads, whole beater trucks, swap meets, etc...