Basically, you can't.
Guide dogs are trained by charitable programs such as the Seeing Eye, specifically for people who qualify as blind. If you are blind, you apply to a guide dog school and if accepted pay a nominal fee for equipment. Some schools retain ownership of the dog for it's life and it must be returned to them when retired.
If you want to adopt a retired guide dog, again, apply at a guide dog school. But these dogs are not sold, just adopted. The waiting lists are typically about three years long and you must pass a rigorous screening to qualify.
There are some Guide schools that do not charge anything for either the animal or the equipment, plus cover the costs for the required 28 days of residential training. Some also offer a yearly stipend to help with vet visits and annual vaccinations.