Every so often we get a question wanting to know where a wheel cart for pets can be bought. On GMA they featured a man that designs and builds them for animals.GMA showed a Lama, rabbit, goat, dogs and cats using them. His business is called Eddies wheels and they are located in Deerfied, MAss. Here is a link to the sitehttp://www.eddieswheels.com/html/our_story.html
If I can find blueprints or a site that features blue prints I will post that as well but so far no luck. If any of you have a site, that shows how to build one please post it. I am sure someone that is in need of this info would be grateful.

Here you go:

Eddies Wheels - Wheelchairs for Handicapped Pets (http://www.eddieswheels.com/)
doggon (http://www.dogwheelchairs.com/)

I reread your question, this will help a little more, I hope:
Homemade carts for dogs and do it yourself wheelchairs for dogs (http://www.handicappedpets.com/HomemadeCarts.htm)

Why don't you do a search here and find many of answers where I have suggested K-9 Carts - Wheelchairs for pets, pet wheelchairs, dog carts, mobility aids for disabled pets, pet products, services, and support (http://WWW.K9carts.com) and Eddies Wheels - Wheelchairs for Handicapped Pets (http://www.eddieswheels.com) in the past?