Not gonna happen. No USB printer will work on a PowerBook 520c short of connecting the USB printer to a newer machine that supports it and possibly using some sort of printer sharing software to share it to the 520c. (the ideal situation would likely be getting a printer that has Mac OS 9 support, connecting it to a USB Mac running OS 9 and using Apple USB Printer Sharing)

Modem and ethernet cards are available on eBay, however you'll want to make sure that you're getting one that has Mac drivers. If there's a card you're looking at getting, go to the manufacturer's website/google the crap out of the name and see if there are any Mac drivers before you buy it.