These are general BDSM equipment links. If you want links specific to pony play, check out my ponyplay links page.
The links below are to retailers that I really like either because they make something special (e.g. pear gag), and/or they are high quality, or even just have a "look" to them. It's certainly not meant to be all inclusive.
Fetters - Really high quality products.
Fleet Ilya - Harnesses, cuffs, collars, masks, and more. Most of their stuff is beautiful and of the highest quality (albeit quite expensive). There are a couple things that are not quite to my taste, but are still well made.
Rupert Huse & Son, Inc - Superb quality BDSM gear and some unique products like their pear gag, which is, to my knowledge, the most accuarate historical replica of the medieval torture device of the same name.
S Art - They have a few cool items including a metal pear gag and a ball gag with bristles.
Medical Toys - Not just medical toys, but other BDSM gear as well.
Winter Fetish - Their products are high quality and include armbinders, catsuits, hoods, sleepsacks, and straightjackets. Their zippered leather armbinder is one of the best I've ever seen - or tried :)
Erotik Elektro - A good selection of electrostimulation products.
BDSM-Laden - They have some cool looking bondage gear and e-stim gear as well. The site is in German.
SM-factory - They have a nice bit gag/ball gag combo (a ball gag on a rubber coated metal bit) that holds the ball more securely in the mouth. A great option for gagging ponies!