I see used external modems in the want ads all the time. Just about anyone will probably work if it is a modern 33k/56k version. A lot of PC owners have dumped them for cable modem or DSL or internal versions. You could probably get one for practically nothing. If you are looking for a list of compatible modems, simply install Miami v3.2 and look at the list under MiamiInit (these are the ones that Amiga compatible version strings exist for) and go from there.