Where can you buy the safety caps for camera batteries?
 

I find myself with a battery that doesn't have a cap and my Google-fu isn't up to finding anything of the sort. Everything that comes up is either the battery or the battery door for the camera.

What type of battery and what are you looking to achieve?

If it's to protect them from shorting during travel, I'd probably get something like
The clip on protectors that come with my Canon batteries can be knocked off quite easily.

Panasonic Lumix DMW-BLC12PP.

It's what I usually take hiking. There's little risk of the cap being knocked off the way I carry them. The contacts are somewhat shielded but other metal things have been known to ride in the same compartment.

I am going to look at those boxes for my Nikon batteries. The caps are less secure and they normally ride less confined.

Would buying a pack of party balloons and cutting them to fit work?

Fuji NP126 batteries fit in a Tic Tac box. :)

I ordered one of these and it's great. Had it since 2017 and it's nicely broken in. Love it.
https://www.etsy.com/listing/5037902...ef=shop_review

I use stretchable, plastic food wrap. Water proof and short circuit proof, and cheap!

Both of these are single-use approaches. The caps on my Lumix snap on so they're very secure and they can be used again and again. And they're not likely to blow away in handling.

A balloon can be reused multiple times, acting as a non-rigid cover. Cut it open and you can put the battery in, like a foot into a sock.