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.
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.
RAM FX Studios - They have some pretty cool looking latex masks, including the punk horse. They also do some custom work for latex pony masks and their prices seem quite reasonable (the punk horse mask is $180).
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.
Bully Bros Gear - Two puppy bros with an affinity for leather team up to make stylish and unqiue leather animal role-play hoods, collars, leashes, and bondage gear. They do not do custom work, but they occasionally have pony hoods and regularly stock puppy hoods.
Tromper's Pony Play Gear - They make cool pony hoods, hooves, and tails. They have do custom work and periodically have pre-made pony gear, including hoods, for sale.
Baron Vulkenstein - Cool rubber pony hoods! Link goes to their twitter; contact them directly for ordering.
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.
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.