3290's are not super common. They do seem to be more common than 3270/3278/3279's though.
Someone in this thread mentioned the microcode floppies for the 3174. When getting mine running, the software was tied to the hardware so it was free. I just called up IBM and they shipped me a full set - in duplicate (one for master, one set for running). I've no idea if they still provide it though. J
