While prices used to be lower, these were never clogging bins at thrift stores.
So very true, even at the height of visiting thrifts in the late 90's/early 00's I only ever saw perhaps four TG-16 systems and one was missing the back cover and hookups. Of course everything before Playstation is impossible to find now. I second the Everdrive option if you are not sure how far you want to take collecting for the system (I do not own an Everdrive).
My best place for games has been with system lots on Amazon. Both of my TurboExpress units were bought broken for under $100 and came with a handful of games. I like having a spare system or two, however. Also check out half.com, most people forget about it and I have snagged a game or two from sellers in the past year. It's not a lot, but it is something.
You should also make a personal want list for the system, the best titles that mean a lot to you personally.