This site is often recommended on a sled dog list I'm on for bootie patterns:
http://www.uberpest.com/booties.html

I suggest using cheap polar fleece for the bootie part and some good velcro. Booties don't stay on very well so they rarely get worn out. They get lost in transit!

That's actually why I don't buy them. There are enough other mushers around here, that I can usually pick up enough the trail for my dogs! I don't use them very often anyway. The only ones I've bought were for Founder last winter when he split a pad wide open. He has massive feet and none of the ones I'd pick up were even close to big enough. Silly sprint mushers and their tiny dogs!

Have fun sewing! The i-rod dogs and their mushers will appreciate your efforts!