Search either AliExpress.com or Ebay.com using the search phrase “Your-Screen-Model-Number Control Board”.

If your iMac is a 20", they should be around $30-$40. If 24", $60-$80.

Order the video driver board for your screen. It will probably ship from Asia, and take 2-3 weeks to get to you.

It probably will not come with a power supply. This is normal. Most video control boards require 12v 5A power supplies. You may have one in your spare electronics bin, so check there before buying one: Old routers, PC laptops, etc may have compatible power supplies.