OPTION 1) Hire a trainer for a dog you have.
The minute your dog is trained to perform its first disability-mitigating task and is in complete control in public, it is a service dog. However, a great deal of work is required for this preparation. An Internet search should reveal service dog trainers in your area. If your current dog has good temperament and aptitude, it can probably be trained for a wide range of tasks that can mitigate disabling anxiety or depression.

 

• Costs 
  Costs for for service dog training vary widely, but PTSD service training can be as low as $1,000 or $2,000, depending on the tasks, the dog, and the trainer, whereas other kinds of service dog training (such as guide dog training), can cost as much as $20,000. For a psychiatric condition, you could have your dog trained over time and add more tasks as you can afford it. Also, you could hold a fund-raiser or start a GoFundMe page.
   

• Trainers 
  Trainers vary widely in styles and personalities, which will have a strong effect on you as well as your dog. So, interview them thoroughly and go with your gut. Ask what tools and types of training will be used. For example, will there be only positive reinforcement? Only negative? Both? Will the trainer use treats, clickers, headcollar, choke collar, prong collar, shock collar? Many trainers warn against use of a lot too much negative reinforcement, and no one should ever hit, shock, traumatize, or yell at your dog--or even raise your voice. There is no need to if your dog is suitable for training (which a professional trainer can verify). My trainer used "military style training," which involves low-key positive and negative reinforcement. He did not believe in using treats, and there was no need for extreme tools.

 

OPTION 2) Find a dog that’s already trained.
Some non-profit organizations raise and train service dogs for particular tasks—for example, K9s For Warriors provides service dogs to military veterans with Post-trauatic Stress Disability, traumatic brain injury, and/or military sexual trauma as a result of military service post-9/11. 

OPTION 3) Train a dog yourself.
There is no law that says you can't train your own dog for service if you are able. However training can require a great deal of research, learning, time, and commitment. YouTube videos can help, and key personalized sessions with a trainer can be invaluable.

OPTION 4) Try a combination.
Some people complete the obedience training themselves and then hire someone for service training, bringing costs down.