other than ebay... you can get them from ford racing. If you go to your local ford dealer ship they should be able to order you a set(but it will probbaly cost you an arm and a leg). or you can go to a junk yard and find a v8 explorer and pull off a set of iron ones and get them rebuilt.
The parts shop i got mine from(which i would recommend to anyone looking for anything ford related)
http://holcombmotorsports.com/
has them complete for under 1200 and the gt-40x's also for under 1200
other than that i dont know of any other place to get them.