Look for FTDI chipset based usb to serial devices. avoid Farallon or other cheaper generic chipsets. You will have to do some research, but the manufacturer should be specific in listing these details.
I have never used these types of devices in OS9, but I would imagine they would work just the same, worth a shot.
Another advantage of FTDI is that they list drivers for OS 9 here at the bottom:
http://www.ftdichip.com/Drivers/VCP.htmThe download seems to work, haven't tried installing myself.
In OS X, ive had quite a bit of success with FTDI based usb to serial devices for doing things like file transfer to my Apple IIe via ADTPro, communicating with Arduino and other serial devices...etc.
Hope this helps.