I guess Walmart has some that is just clay... It is Walmart's brand, Special Kitty litter... Hope it doesn't have lead! Jk, a scientist on the forum did an analysis, not very much if any, I forgot lol.
Coolers can be pretty expensive. If you know someone who is handy you could ask them to make one from an old water fountain. I have to use some cooling techniques on my goldfish tank in the summer to keep it below 80F. I just leave the lid open and always have a fan going.
I am not sure I need to find something small that will be fine in the 16 gallon. The daces seem quite nice. If I do decide on going with native species, I will need to research more though. That is if I do score to go down that path.