Where can I find some antivenom?

You can create it, but otherwise it's mostly found on legionnaires.

Traders have some from time to time.

Get your survial up high enough or use a book and make it at a campfire, I think I used mentats too. You need nightstaker blood ... dangerous to get but rewarding since you can make antivenom. It made 6 doses, so I got like 9 now including the ones I found.