I buy them off ebay at the moment but any standard rubber bands will work. My favourite brand is "5 star office". £5 gets you a bag of over 300. I use 16 in each bandset so plenty for that price. The band size is #64. % natural rubber so nice and snappy.
They are surprisingly good shooters and cost pennies to make a bandset. You have to shoot them "active" (release the ammo on the pull back rather than holding an anchor point). No rotary cutters, no mats and rulers. Just chain them up and fix them to your catty. They really throw heavier ammo well. Lead, pebbles, big hex nuts are g