As previously stated, if it's only the glass that is broken then any good auto glass shop should be able to replace the mirror glass at a reasonable price.
However if some of the mechanical parts behind the glass have been broken, you will need to get a replacement mirror.
If you can get the glass out, you can often find precut replacement mirror glass for a limited number of vehicles at some auto parts stores. If you look for Eplorer mirror replacements, you may get lucky and they will fit, but I doublt that you will find any labled specifically or the Sport Trac.
...Rich