It has always been a little difficult to get a USB modem for a Mac but with the added disadvantage of OS 9, it is even more difficult.
As far as I can see, only one company are making them for OS 9 machines
Best Data: Best Data Smart One 56K USB. They cost anything from 49.99 from one site and $60.00 from the company site. Both places claim that it works with OS 9.x
Apple sell one at the Apple Store but it only works with OS X. As you may have noticed, virtually nobody makes stuff for OS 9 any more.
http://www.costcentral.com/proddetail/Best_Data_Smart_One_56K_USB/56USBPMAC/813707/froogle/ is the link to the cheaper site

Hope this helps

P