OP: I honestly don't like the "kill or keep" talk here. The important thing we have to make clear is that if your dog is in pain or is having seizures you may want to seriously look into ending the situation. But if your dog is simply parylized and will never use it's hind legs and is NOT in pain there is hope. It makes no sense for some people to recommend putting the dog down because their dog was in pain if your dog is not in pain. I can tell first hand, I have seen a couple of these dogs at the dog park getting their exercise. They are playfull and still run and play a lot more than you would think.
Here are some links for you
http://www.k9-carts.com/
http://eddieswheels.com/
http://dogswithdisabilities.com/wheels.htm