In Germany, I know of these suppliers:
Banux (
http://www.banux.de), which has been mentioned here before. At the moment I think they only have cattle skins (cow or bull/steer from Southern Germany). They offer them in two options: natural treatment and polymer treatment, at a moderate price. I recently bought a skin with polymer treatment for a bongó hembra that I am very happy with.
Altenburger Pergamente & Trommelfelle (
http://www.pergament-trommelfell-shop.de). I haven't had any personal experience with them, but I have got good references from different sources. They offer skins from a few different animals: cow, bull, calf, horse, deer, goat...
I also know of a drum restorator who has offered me top-quality thick Brazilian cow skins. I don't know if it's alright with him to have his contact published here, so I'd rather ask him first.