I've done a couple of these lists on bridles, and saddles. I was holding off on doing one about masks/hoods because I was hoping to find a bunch of "secret" retailers somewhere that kept their products hidden. Well, I was not successful in that respect, but I figured I'd list the few I knew about and hope that you guys could let me know of any I missed.
I'm not including the meme-inspiring, crazy-looking horse head (though if you want one of those, you can get it here). Incidentally, I've seen those particular horse heads turned into decent looking zebra heads (though undoubtedly with a fair amount of time going into the paint job), and likely you could paint over the whole thing with black paint to get a less crazy looking horse, but I digress.
Anyway, onto the list, which is in no particular order:
Fury Fantasy - They have a few different styles of masks (i.e. "minishetty", "FuryFe", and "horse mask"), all of which share the common attribute of being uncannily realistic. A minishetty mask can be had for €989, which, while certainly not inexpensive, is actually not too bad considering the quality of the finished product. Their FuryFe, which is still considered pony size, goes for just under €1300, and their full size horse head mask is €1599, or just under $1800.
Goblin Art - A wide variety of masks, which tend toward the more artistic and less the fetishistic (e.g. this wonderful fox mask). I stumbled across her site not long ago, and some of her work is breathtaking. From reading through her site, it sounds like she does custom work, but also sells at various fairs and events.
Bob Basset Studios - Bob Basset has been known for his excellent craftsmanship and attention to detail. His leather horse masks have a beautiful look to them (e.g. this brown one). His studio has continued to make hoods following his death in 2011. Since I don't own any Bob Basset masks, this is heresay, but there have been reports that the newer masks deform more easily than the older ones (though the newer ones still certainly look good, see photo at right). I recently purchased and reviewed a Bob Basset mask, and I can say that the quality remains impressive even after his death. The leather mask does not appear to deform easily, and the mask retains its shape under most conditions. I believe most masks are sold through their eBay store.
Wild Gas Masks - Wild Gas Masks does beautiful work. They work in latex and have several base models, and they will do custom painting and coloration on their masks. They had a unicorn model, which sold relatively recently. I saw it live on the awesome pony who bought it at the 2013 SF pride parade, and it was truly stunning.
Secret Latex - A full latex rubber horse hood for £55 (around $80). While it is a basic horse hood, it's a great place to start and decide if a full horse mask is right for you. The mask has a far better look than the crazy horse head meme masks. However, I have been told that it is not quite sturdy enough to hold a bridle in place if that is one of your goals. Nevertheless, it is a great way to get that horse look without shelling out a fortune.
This is an even shorter list than with the saddles, but hopefully it's still helpful. I'll add to it if I learn of other retailers that make them.
The really cool thing about having a horse mask is you can use bio-horse bridles and halters. For bio-horse owners who obsessively buy tack, this is the equivalent of discovering that your well-dressed roommate with a huge wardrobe is, in fact, the same size as you :) Seriously, I love the look of all these masks, and I love that we have such talented artists.
If you know of other retailers of horse head masks not listed above, please do contact me, and I'll be more than happy to include them on the list so other ponies can easily find them.