My very first job as a graphic artist was for a direct-mailing company that was located near two grocery stores and two gas stations. All four of them had a vending machine that sold these. We had a massive collection that I don't remember who started (or what happened to it) in our office. Just hundreds of them lined up all over the place. They were cheap so you could get them easily. I want to say 25 cents a piece. Grocery stores and other places used to have these little, mostly red, rows of small machines that sold candy, toys and stuff like that, always in these strange plastic bubbles. I dunno if you could buy them any other way, but that's how we would get them.
If you really want to go down this rabbithole, Vice did a fantastic short documentary about this stuff...