Keep an eye out on eBay but in my experience you won't be saving much, if anything, over buying the whole shebang since sellers tend to factor in the time and effort taken to dismantle and test boards. In fact, Logic Boards often go for several times the cost of their respective Power Macs.
You might try to find another Quad with broken CPUs or a severely damaged case and try to extract the Logic Board from that and hope that it still works.