Very common and cheap (around the $4-6 mark per fish)
and LFS should have them in stock
during summer they breed in outdoor ponds and tubs (I had some breed in a big pot I had outside)
other good option for your axo is leopard fish, just set up a small breeding tank (25L or so should do for a male and a couple of females) next to the axo tank and send the fry to their doom, its not cruel, its just nature with a helping hand holding a net