Look around on the speakers for the rating. Most are something like 12VDC. Then you simply need to find a similar power addaptor. You should also see something with an mA rating, but as long as the new addaptor is the same or more, you'll be fine.
Where to find them? Radio Shack might have some. If you live around a place like BigLots they might have some old ones laying around. Etc...
Once you have something, you should be able to just cut some wires and connect the plug from the old wire to the new one. You'll need to worry about polarity, but that's as easy as just switching the wires around...(weither the + or - is on the inside or outside of the plug)