Search for "5V/12V/24V Universal LCD/LED Power Supply Power Board" on your favorite search engine. I found mine on ebay.
The original Apple PS can be used, however. The 24v output is always on; use the original cable with the exception of the gray and white pair. Route those from the new video card to the positions as shown in this image: https://a4.pbase.com/o12/88/791988/1/169904074.9dumKy8s.07-InverterPower.jpg. You can also use the 12V output on pin 5 of the large connector on the original PS to drive the video card.
See my post on iFixit for details: https://www.ifixit.com/Answers/View/213674/iMac+dead.+used+as+a+monitor+lcd+panel