I'm a big fan of Jacquard products. They have a very nice metallic fabric paint, but as far as ponies go I really like their Pearl Ex Pigments. I used True Blue for my customized sparkle pony Sky Rocket that I've shared here before, as well as another color for an unfinished custom, and they add a lovely metallic sheen to things. Highly recommended!
I usually buy Jacquard stuff online from Dharma Trading, they have pretty good prices with reasonable shipping rates. Hobby Lobby also carries a limited selection, and of course eBay is always an option!