Most any RS-232 modem should work, provided you have the right cable like Larry mentioned.
It's worth mentioning though, if you're using a terminal emulator on a Kaypro II (and maybe 10) you'll be limited to around 1200bps, or 2400 if you've got a speed mod installed. This is becuase the video circuitry is slow and the terminal programs can't process incoming characters while scrolling. There's a few solutions out there, but YMMV. This issue doesn't apply to file transfers, which should be OK up to 19,200bps.