You are in luck, your performa has a Comm Slot I and an LC PDS slot. You can buy them at several different online retailers. Check out <a href="http://www.macresq.com" target="_blank">MacResq</a> , <a href="http://www.poweron.com" target="_blank">PowerON Computers</a>, <a href="http://www.smalldog.com" target="_blank">Smalldog</a>, or do a search at [URL=http://clubmac.com], or any other major mac retailer. You also might want to weigh what the the investment in one of these might cost you... for example, at PowerOn, you can get a PM 7220 that has the built-in ethernet, more expandability and a much faster processor for $89.99... so again, way all of your options.
Expansion slots in Performa 5300
LC-compatible 68030 processor-direct slot (96/114-pin)
Communication slot for optional internal modem or Ethernet card
(112-pin) (depending on the configuration of your computer, a modem or
card may already be installed in this slot)