XPS 8700 PSU

Short version:  I need a replacement PSU for my XPS 8700.  Where can I get one?  Or a different one that can be plugged in without any issues.

Long version:  while I was away for a month, it looks like my old reliable 8700's PSU has given up the ghost.  Computer does not power at all.  And if I unplug it for a while, then plug it back in, nothing happens.  It used to be be whenever I would re-plug the computer, the fan and some LEDs would light up for a second, but not anymore.  There is a button and green LED at the back of the PSU, and if I press the button, the LED turns on, the fan powers up, but they both go off the moment I remove my finger from the button, and computer still doesn't power up.  Don't want to replace the computer since it is running fine and more than enough for my needs.  But is the PSU shot?  And if so, what's the easiest way to replace it?