Q.Saving an old Azalea
I have an azalea that is over 10 years old. It was planted with two others beside our house. None grew very much over about 5 years so I moved them to the backyard. Two have died and this one is only about 8 inches high and the dogs broke some of the branches. I’d like to save it and turn into a bonsai. Can I put it in a pot for awhile to get it healthier and what kind of soil should I use?

How does plant grows

It is unlikely that an old unhealthy azalea would make for a good candidate for Bonsai.
You can however repot the azalea to try to return it to a better condition.
Look for a good quality potting mix and a pot with adequate drainage.
Here is a few links for you.
https://www.gardeningknowhow.com/ornamental/shrubs/azalea/common-azalea-problems.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/types-of-potting-soil.htm