@rockm makes good points.
I make a point of listening to him even when I disagree.
I got caught up in the conversation, and didn't think to make the points he just did.
I will place some caveats on them. Looking for decent starter material to learn on can be solved by collecting, but a good rule of thumb is the more you like it or the nicer a tree it is, the more you should reconsider.
STICK TO THE WEED TREES. Leave that beautiful piñon pine alone, no matter how amazing it is.
Collecting sagebrush, mountain mahogany, any invasive species; things like this have decent survival rates, won't be missed, and can make good trees. Great for learning how to care for and style trees without ever feeling like you're wasting your time.
Same goes for the neighbor ripping out his landscape plants. They're going to die anyway, so take the opportunity.
BUT LEAVE THE BEST TREES ALONE.