Look for Power Supplies on Ebay that are for telecom equipment. There is lots of surplus out there and many handle pretty big amps. Many are 48 volts (some can be adjusted slightly higher or lower).
It may take awhile to find the price you want. But I found a 60V 30A (1800W) power supply and paid only $49.99 on Ebay.
Just be careful and stay away from anything that says "3 phase". Also, check and make sure it's 115V-in if you don't have the proper 220V outlets (220V come with different plugs for different amps).
Ebay Search Terms:
Power Supply 48
Power Supply amp
Power Supply CRU (charging rectifier unit)
Power Supply Cherokee
Power Supply Lambda
Power Supply Kepco
Power Suplly Siemens |