So growing a bonsai cannabis tree is more to do with how you grow it. It doesn't seem like there's just any special strain that just grows magically into a bonsai tree. Growing a bonsai tree involves using specific growing techniques to try to control how the plant grows.
Although Royalqueenseeds does recommend the strains Critical Kush or White Widow as good strains to get if you do plan to try to grow a weed bonsai tree.
If you want to learn more about those specific techniques a good place to start is reading through some of the links at the end of this document.
There's actually two whole entire subreddit dedicated to bonsai weed plants
Well Royalqueenseeds recommended White Widow and that first example I found is really pretty and is a autoflowering white widow so white widow seems like a good bet at least.
This guide recommends either MSNL seedbank or Crop King Seeds
There's no special strain required to grow a cannabis bonsai tree, you just have to follow the general techniques for controlling the growth of the plant to make it look pretty and bonsai-y and small.
However if you are going to get a specific strain white widow seems like a good option to try to grow a bonsai tree with.
White widow seeds can be obtained from various online sources, some good options include Crop King Seeds and MSNL Seedbank