scour landy breakers and ebay ect, best to go with a recon engine but if you have to buy used, make sure you can hear it run before you buy and make sure the cooling system doesn't over pressurise (indicating a possible liner problem), even used 4.6 engines are expensive and the price is going up because they're not made any more and everyone wants them for kit cars etc.
Whats happened to yours? can't it be repaired/rebuilt?