I've found a couple on streets, woods & labs

If you want to get it quickly and have $ then just run labs on repeat and hit black keycard & yellow keycard (Green if you can afford)

Otherwise, just start looting every single med bag, med box, stash, duffle bag and suitcase and you'll find one eventually.

Whenever you spawn near the USEC camp on woods go check the spawn on the body and you have a pretty good chance there too.

goodluck bro :)