RCA wsp150 wirelss speakers

Jim, just about any generic power supply will work, so long as you match the output and polarity.  There should be a label on the old supply with the voltage and Amps (milliamps, actually), and a circle-in-a-circle with a + or - that tells you the polarity. 
If you're not sure enough to just buy one from Target, spend an extra buck or two at Radio Shack.  Bring in the old one and ask for a replacement.  You may end up with an adjustable adapter with interchangeable tips, but it will work just fine.  Sometimes the mA are a little different, but that should be okay, so long as it meets the minimum requirement on the label.
I have a pair of these running my WSP 250's.