You can get multi-port PCI NICs - this may be an easier solution. Don't expect them to be cheap though.
Your only other solution is to crack the case on a SOHO switch and mount the board to some sort of modified rails and run the power cable through the case, but that can be cumbersome, time consuming and annoying. If you are handy with electronics, you could make a voltage adapter circuit, going between the +12V line on a molex connector to the connector on the switch. You'd need to be aware of the switch's electrical requirements so that you do not tax the power supply.