Sorry to hear that. I hope you manage to get the funds to rescue some turtles at some point soon
Just a note, I don't know how the humane society keeps their turtles, but don't mix species. You should be okay with a couple of sliders in that tank, 10G per inch is the ideal scenario, but aslong as the water is kept clean and they have plenty of space to swim and bask together, it will be fine
If you happen to choose something other than sliders, such as a softshell species, only keep one. Softshells are very aggressive towards each other and other tank mates, but at the same time very delicate and prone to injury. They are not communal species at all.
All the best, hope the cats are okay,
Paul