Well, you definitely do NOT want a Winmodem, as they only work under windows. But I bet most modems today will not work with an old 386sx dos machine, so be careful.
Here is a
USR Modem that sounds like it would work for you.
I would suggest going to the web pages of USR and Zoom and other modem makers and see what they have. Or, look at modems at buy.com or Amazon.com. Or, go to ebay or some place like that and see if somebody is selling an old modem.
In the specs, you need to check
(a) does it work in dos
(b) will it work with your hardware -- in particular, what kind of slot does it want? And does it want a 486 or better computer? If it is a PCI modem and you have no PCI slots, you're in trouble. If you must have internal, then I believe you want a modem that goes in an ISA slot, but I may be wrong. I suspect your safest bet is an external modem that only requires an IBM PC-Compatible computer, which is what the USR listed above claims as its specs.
(c) Do you need DOS software? Don't know how much you'll find, but check the specs.
If you find a dos modem, the install disk will probably either modify your config.sys and autoexec.bat files or tell you how to do it. If the modem will work with DOS, it (hopefully) isn't going to make you just make wild guesses as to how to make it run. You might be able to, or need to, download dos files from the company web site. Good luck. RW