"prebuilt computer systems" would not be the place to ask. From what I can see the power unit on one of these has no relation to a computer PSU. I'm not even sure it's DC...
On page 82 of this .pdf file I think is a diagram of the needed PSUhttp://www.studionu.com/amr/manual.pdf
I have no idea what I'm looking at, but perhaps some of the smart folks at jonnyguru could help point you in a better direction at least:http://www.jonnyguru.com/forums/forumdisplay.php?f=3
If you ask very nicely and point to the PDF, mentioning the page number, maybe?
I doubt very much that you can just buy something that will work, but maybe something can be adapted since the pinout is also there in the PDF.