A power amp is just the power section of a guitar amp, without the preamp and controls. You use your amp for tone shaping and the power amp for driving more speakers. Here's a Marshall 50/50 on Gumtree, which is two power amps in one (for stereo or running up to four cabs):
http://capetown.gumtree.co.za/c-Stuff-for-Sale-musical-instruments-Marshall-50-50-power-amp-W0QQAdIdZ214950107
...quite a good price too.
You can also use another amp and speakers too (or even combo amp). Simply plug a cable from your send/lineout to the return on the other amp. When you see a wall of amps on stage behind big rock acts like The Who or AC/DC, that's how the amps are usually connected together.
You could also have your amp modded to access the speaker output so you could plug the built in power amp into a bigger speaker cab, cutting off the internal speaker at the same time.
