The trees that you have seen planted in the ground are not bonsais. The actual word bonsai means "tree in pot." There are a lot of beautiful Japanese gardens in the Pacific Northwest. The trees in the ground that you are admiring do not technically have a name. They are simply trees that have been trained and pruned using the same techniques as that for a bonsai in a pot. The training that it takes to do this is very technical. It also requires special tools and varying sizes of copper wire.
This would be an excellent ongoing hobby for an individual interested in learning the art of bonsai. Because this type of gardening is popular in the Pacific Northwest, I'm sure that there is a bonsai society or club near you. I would contact them and ask to join or ask about bonsai classes they may be teaching.
As you may know, a bonsai is a type of pruning, not a specific variety of tree. Many types of trees can be used as material for bonsais. Some trees to think about using are:
- Japanese maples
- Various pines (white pine, mugho pine)
- Maples (red, white)
I highly recommend reading Bonsai: Special Techniques by the Brooklyn Botanic Garden, Keep Your Bonsai Perfectly Shaped by Herb L. Gustafson, and Miniature Living Bonsai Landscapes by Herb L. Gustafson.