Collecting trees in the wild is illegal in many places. It's not a practice that I would personally endorse. Most bonsai enthusiasts grow trees from seed. There are quite a few that will train nursery stock, though.
As I said previously, bonsai trees are not starved of resources. Quite the opposite. They are specifically pruned in a way so that the tree is able to absorb nutrients much, much better than they would if left unpruned.
You are probably wondering why they trees are small if they aren't being deprived of something. I invite you to spend some time learning about horticulture if you are truly interested in the answer.