how do i buy a genetic tester?

Unfortunately aside from the Infinite one obtained from the Diamond Shop, you can't buy testers or scanners; It used to be possible to get scanners and testers from the Wheels during fests, but the Wheels were retired, so the only actual way to get one-use ones- other than the newbie packs- vanished.

However, if you need pets that you own tested, there are many people who offer free pet testing services in the Groups, Guilds, Clubs, and Services forum.

After you've used up your freebie one-time tester from the game, really the only way to get more is to buy an unlimited test kit from the diamond shop.

They do go on sale during fests, but they can still be pretty expensive (if you're not a dia buyer).
Dias can be bought with gold from other players, though, on the Currency Exchange.

If you don't want to buy one, you could check the "Groups, Guilds, Clubs, and Services" forum. Many people with unlimited kits are willing to test other people's pets for free.

(oops, Scath beat me to it)