I just googled this site. Maybe it will help.
HandicapPets.Com
Products, services and support for special needs pets
http://www.handicappedpets.com/
If You Care for an Older, Injured, or Disabled Pet, You Have Come to the Right Place! A Dog Wheelchair & the 250+ Pet Health products will make them mobile again ready to Play & get the Exercise for a more Happy & Healthy Life.
Help & Support Community
There is a man on ebay who makes and sells the pet carts at a deeply discounted rate. He is doing it because he needed one for his pet and couldn’t afford what was being offered so he started a foundation and began making affordable pet carts for handicapped pets. Should be easy to find.
Much luck and many blessings to you and yours.