on Ebay, you could be buying junk, or an external modem without a power pack, etc.

But yes most brand name external modems will work, just work out the Init string to use with OS/2, usually the same one used with Windows, DOS, Procomm, BBSes, etc.

USR/3COM are the best for external modems. Sportster is the cheapie USR brand, Courier is the more expensive one.