Good job on the mod. It's always a little scary cutting and soldering your new baby. I bought mine that someone else had already done.
Get one of these for the power supply. I believe this is what everyone calls "The Wallmart Battery". They run about $30 plus shipping in ebay. Just swap the wires on the cable if nec. Make sure you get one whichis 9V, not 7.2 or 12.
They seem to come in a couple varieties... for instance, a 5400mah Lion battery, and a 4500mah NiMH. I have one of each. I think the 5400 Lion is better, but they cost a bit more. I have run a UA-5 at a festival for 10-12 hours on the Lion battery, and 8-10 hours on the NiMH. I don't know how much longer they will last... that was the end of the day. Bottom line, you probably don't need 2... they will last all day.