Why not order from the online Apple Store? $49 (here in the U.S.) is not bad for a modem guaranteed to work with your iMac.
If you just can't wait, BestData and Zoom both have inexpensive modems that support Macs. To see what your choices are, I searched with the string "usb modem macintosh." See what you get when you search on that string.
Since your profile doesn't show where you are, I can't tell if you need something other than a U.S. standard modem, so you'll have to do some looking yourself.