Once you get the old seal out any decent seal supplier, or gearbox place can get it, ford might even do it.
I would suggest a seal place is best as then you can get a VITON seal, this is a vastly better material than used as std(by all car makers) and so will cope better as autoboxes do get very hot.
Another thing that would help immensely is the fitment of a air to fluid cooler for the trans fluid, the cooler it is the longer it lasts.
tabetha