How to access internet on an Apple iBook G3 without Airport Card?
That is going to be tricky because any new usb wifi adapters are probably not going to have the driver support with the OS you are going to be using. 
(If you are right about the model the maximum OS is 10.4.11.)

Amazon has original airport cards for sale and it looks like they aren't too expensive compared to newer adapters (~$20). That's the solution I'd use to get that computer online.