I assume that you want to use a computer PSU. They do only put out 12V (I was hoping to correct RickRick), but all chargers (that I have seen - and I have seen a lot of chargers) take 11-15V as input, so a 12V PSU fits right in there and will be fine. It just needs to put out a decent amount of current, at least 12A, if you want to charge 2 largish 7.4V packs at once.
I've just read the CellPro's manual, and it requires 12-15V, so the PSU will be fine. All computer PSUs will only output 12V, as all the components work on this voltage. If you want 13.8V then look at
these. They also output up to 20A, so will easily support charging two 7.4V packs at 8.8A each.