Comet TV
An open air "check you local listings" channel that plays old science fiction shows and movies (kinda like back SyFy when it was the Sci-Fi Channel). I bring it up because they've got some movies that played on MST, and some worthy of it.
Last month they played Alien from LA some, and this month they're playing Robot Holocaust and Squirm, and there have been other films that would've made for decent riffing.
So check your local listings or the Comet TV website to check out some MST3K fodder by themselves...and some that would be good for the reboot