audio input to ibook G4

I need to interface my ibook G4 to an external audio input from a radio, however there are no "mike in" or "line in" jacks. A comment elsewhere on the web suggests that apple does a usb adaptor, but I can find no indication of this on apple's or other supplier websites. I don't want a usb microphone: I need to feed in signal. Any advice appreciated! Regards, Chris.