DA, I read that as they have the hide/skin but not the carcass. While it's a strange request and one I don't think I could have done (though we do have deer mounts) the chances of finding a foal that age at this time of year may be a challenge for you Inky. The majority of foals born for the year are already on the ground. Most breeders try to have mares foal anywhere from Jan 1st (if planning to be young show stock) to late spring because registered horses all have the same birthdate of Jan 1 with breed associations for show purposes so an August foal would be 8 months behind on growth compared to the January foal.
The only option I could think of that may help you would be to contact vet schools or local horse vets.
Life is like a camera. Focus on what's important, Capture the good times, Develop from the negatives and if things don't work out, Take another shot.