Depends which country you are from or which continent...
There is a lot of offers from Germany/Austria
IBM pSeries RS 6000 7026-6M1 4x750MHz 4GBRAM 583GB HDD | eBay
Put "rs6000" in search and check in google models if you do not have experience. Normally everything over 600Mhz it is power4.
Just check to make sure
For power4 you should not pay more then 600euro. Does not make any sense to bay power3 and older.