There are several options:

1. Applying and getting a trained dog from a non-profit organization. You can apply to get a dog from anywhere in North America that are associated with Assistance Dog International or International Guide Dog Federation (Some guide dog organizations also train service dogs for autism or hearing). Some require you do some fundraising to help offeset the cost of the dog and training it. Some don't. Most have waiting lists.

2. You can purchase a trained or partly-trained dog from a private trainer. If you can find one who is a professional member of a training organization and follows a code of ethics, that is good. Also make sure they have a local business licence. Hire an another independent trainer who can read dog body language to evaluate the dogs they offer. Some methods are not suitable for some dogs and that will affect how comfortable and reliable the dog will be and how much stress the dog is under when working.

3. Some trainers offer board and train, but do a thorough check and get references and ask many questions before you take your dog to them. There are very few positive trainers who do board and train (but there are more all the time). Most use balanced methods that may not be suitable for your dog. If you cannot be there to watch, you have no idea of what methods they are using on your dog. I prefer the ones who board the dogs in their own home rather than a kennel environment so the dog won't be isolated or get culture shock when going there and coming home. If you can find one who is a professional member of a training organization and follows a code of ethics, that is good. Also make sure they have a local business licence. 

4. You can get the help of a professional trainer who can help you select a puppy or adult dog and train one yourself with the help of a trainer. If you can find one who is a professional member of a training organization and follows a code of ethics, that is good. We belong to Vancouver Island Training Association. Also make sure they have a local business licence. We have one in the City of Nanaimo for example.

5. Or some combination of the above.
You might be able to get a partly-trained dog from both organizations and private trainers. Then finish the dog yourself.
Or you might start the dog on your own until he's 18 months and get a professional train to finish the dog to public access standards and train tasks. Or you may send the dog for a shorter board and train. Or the trainer might teach you how to fine tune the behaviors and tasks to the level needed for public access. 

In all cases, it is going to cost you money. How much depends on how much of the work work you do.
Avoid any company or organization who offers a guarantee on training. Dogs are living beings that are not perfect. Events may occur that are out of your hands such as your dog suffering emotional trauma from getting attacked or medical conditions that affect their ability to train. 

Can You Give Us a Better idea of What it's like to Owner-Train?
To help you decide if training your own dog (called "owner-training") is for you, I have several blog posts that may help. It's a big undertaking and is equivalent to taking on a child for at about 2 -3 years until the dog is ready to be a partner.

One of the key things most organizations use to rule out people is if or not they have had a dog before. This helps the to have realistic expectations of what it is like to live with a dog at minimum and how to care for one.

Then there is the specific training for public access and the tasks on top of that. Dogs are individuals and are not perfect. They have needs just like people.