Where can you buy a guide dog?

Answer 1

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.
There are many organizations that train them for blind individuals. A search on the internet will yield many across the US, I'm assuming there are also similar resources in other countries.

Other people owner train guide dogs or arrange private trainers to train the dogs for them or some combination of the two.
To get a guide dog you need to be blind or have a severe vision impairment. Guide dogs can be obtained from different foundations (Guide Dog Foundation For The Blink, Royal Society for the Blind, etc.) that breed, raise, and train these magnificent helpers and family canine friends.
A person can sign up to get guide dogs from the UK/US Guide Dog website. A person fills out an application, phone consultation is scheduled, a home visit is scheduled, if application is accepted, one begins classes to get paired with a guide dog.
There are several organizations world wide, but Guiding Eyes for the Blind is stationed in Yorktown Heights, New York, USA.