stir plate question

So we have recently replaced our computer and the wife has given me permission to use the old one however I choose. I'm going to turn it into a stir plate. I understand the directions pretty well but have a question regarding the power cord.
Could I just use the power supply off the old computer? The fan is currently plugged into it somehow already. If I wanted to add a rheostat it would be spliced into the cord from the fan to the power supply... Right?
If I use the power supply off the computer, would it be a waste of electricity or would using a phone charger be a better use of electricity?
Thanks,
Scott

i would just use a cell phone charger.

Yes you can use the PC power supply
Yes it will use more significantly more power than a simple wall transformer, even if you only have that fan plugged in and running.

Well you *can* but its way overkill. That and you will have to figure out how to defeat the soft power switch present in all newer (talking made after 1990 or when ever ATX came out) psu have. cell phone charger is way easier. If you don't have one just head to you local hotel/motel and claim you lost yours after a night of ... well what ever.

Or you can just by a 12V DC power supply on ebay for $0.99 shipped. I went that way with mine. It took about 3-4 weeks to get here from china but if you can wait.. its basically free and takes little to wire into the stir plate.

Our cat water fountain is about to be declared dead (why yes, we do have spoiled cats) and that power supply is 12v. It sounds like that would be a better choice.
thanks