Problem solved. Get a Keyspan PCI USB card and, as long as they're running OS 8.6 or higher, they can use any Mac compatible usb printer on the market. Put the card in the machine, and it gives them USB capability. The card is about $30, and USB printers, like the Epson C60, etc., run anywhere from $80 on up.
There is no adaptor to turn the serial port into USB, so this is your only option.