Please remember that I also have a lot to learn, but I'm willing to share what I know.
Generally speaking, getting a nursery tree saves you years of growth time, but also needs years of development (which is half the fun, in my opinion). Of course, you can get a more finished and expensive bonsai and always continue development, like
@rockm did with his wonderful oak tree.
Things to look for in nursery stock:
1. Species that is good for bonsai, and native to your area or grows well in your climate.
2. Look at trunk and nebari. Do they still need years of growth?
3. How long will it take to end up in a bonsai pot vs how much time are you willing to spend on it?