Where do you buy trees?

[Prin]

I'm thinking of getting a small birch tree from in back so there is something blocking my view of the neighbor's balcony, but I can't find one. We tried nursaries, but they have maple and bushy trees.
Do I have to go somewhere more specialized?
When do you plant a tree? Does it matter?
How can you tell if you're getting a good one?

Anybody ever bought a tree before?

(sorry for the 20 questions, but I want to research before committing to a tree)