The power supply on my 4-5 year old Dell Precision 490 failed about 2 months ago.
I found a reasonably priced refurbished unit from http://blueskyave.com
The new one has also now failed and blueskyave has a 90 day warranty so I emailed for a RMA # and they issued me one.
I have returned the failed unit to them and am now waiting for my replacement.
This seems to be a very reliable company for Dell parts. Will let you all know how the replacement goes