Does anyone have any experience/recommendations on getting a therapy dog?

I'm talking about a real trained one not just some piece of paper I can get online for any dog to bring into Target.

My son gets anxiety sometimes especially at night and sleeping alone in his room and I'm seriously considering a therapy dog for him.

Is it even worth it or you think any dog would help in my situation? I would love to adopt, but I also have a toddler and all of the dogs at the humane society are pit/lab/shepard mixes which are all high energy mixes and I'm not a high energy person. And I get nervous with those breeds not knowing where they came from and having a toddler. She already annoys the crap out of our cat.

Does anyone know how you get one? Do you get the puppy and then pay for training? Or can I get one that's already trained locally?