Whitening and croaker from my local Walmart. They are already headed and gutted so I just stuff the cavity with turtle food and croc chow. Whatever whole fish the right size I can find for a good price I snag. Fish average 3" deep, and 8"-10" long for the big two. 4"-6" long and 2"3 inches deep for the little guy if that makes sense. They typically eat 2 or three at a time. If it fits in their mouth and down their throat they will eat it.
I was fortunate that the bigger two took to frozen fish right away. My smaller one is still a work in progress.