where to syphon off voltage from a 25 pin serial?
You should be able to use pin 7 for negative, 9 as positive, this gives you 12 volts.

YOU MUST NOT HOOK THIS UP DIRECTLY to the arduino, connect it to a plug for the power jack (right of the USB connection).

If you are using a USB -> serial adapter, then the voltage may be 5v, but don't count on it.

Of course, USB is always going to be better!

PS - if you draw more power than the serial port is rated for, you will probably destroy the card. In most cases, this will be the motherboard. You are unlikely to be able to find out what it is rated for (look for Amps or A, or milliAmps  mA, or microAmps uA). The Arduino (with NO inputs or outputs, and idle) running from a 9v battery will draw about 50 milliAmps.