Where can I buy a genetic scanner? I think that is what they are called. The ones that can scan Sylesties and enemies. Where can I buy one?

you can't, except for the Infinite one that you can buy for diamonds in the diamond shop (which IS 100% worth its cost, but also, is expensive)
(does this make sense? No. It used to be that you could get a stock from the festival wheel, but the fest wheels were retired a few years ago leaving no way outside of like, the advent calendar to get basic ones, and all advent calendar items are non-tradeable)

Youre talking about two different things here, I think- Genetic scanners and genetic test kits. The former gives you the genetics & statistics (Health, Agility, Intelligence, etc.) of Sylesties you encounter in the wild, like in the festival zone. The latter reveals only the genetics of pets you own, buy, etc. Both come in standard/basic and exceptional forms. The standard ones can be found on the trade broker & bought with gold, and can only be used once. The exceptional ones cost a type of in-game currency called diamonds (Which, themselves, have to be bought with real money or traded using gold.), and have unlimited use. I actually have three standard genetic scanners in my inventory if you would like one or more of them. Let me know either way.

I already have the exceptional ones and I happened to still have the free standard one that you first get in the game. I'm never going to use it because i have the unlimited use ones so i just sent over the single use scanner