I used to scope out material from mom and pop old landscape nurseries. But as they've gone under or retired, or given way to the mega hardware store landscape places, I've lost that avenue.
I've since bought most of my nice material from either NEBG, Gregory Beach Bonsai, or Wee Tree. Starter material mostly from Ebay. If I just want a learning tree, (like pine) I have gotten it from ebay, and not spent a lot to learn on it. But you almost always get what you pay for....
Don Blackmond from Gregory Beach is a great resource for me for a wide range of material
NEBG has some beautiful well developed material, and some interesting variety.
Wee Tree has less developed, but good potential trees, their bare root stuff is beyond compare for the price.
I have always wanted to buy from Teleperion, I hear such good things about them, perhaps next year.
It always helps to know what you're in the market for, as that can have a bearing on who you look to for that type of tree.
I tend to just ask around for what my resources have at my price point. Then you can compare what the best tree at the time that you can afford is.
Hope this helps.