Search the forums for Windsor to Romeo swap. Tons of people have done it (including myself).
Search car-part.com for a list of salvage yards in your area. Look for the motor from a 2003+ Grand Marquis, Crown Vic, or Town Car to get some sample prices for whole salvage yard motors that can be swapped into your Mustang. While there you can look for motors from a Mustang. But in general you will find these to be more $$.
Hint: Avoid any motor from a former Police car. They spend sooooooo much time idling that mileage is no longer a good yardstick of wear.