Fuji's excellent instant films, 100c, 100b and 3000b, will all work in that back. Make sure you get the 3 1/4 x 4 1/4 size and not the 4x5.

Fuji instant film is readily available online from Freestyle, Adorama, B&H, Ultrafine, etc. Here in the San Francisco Bay area, there are at least two retail camera stores that keep it in stock.