I took a bunch of young from under a friends tree as they are (supposedly) very invasive. You probably wont find them for sale due to this fact. I haven't had any luck in getting young, yet. Might be due to the thick grass under mine. I have some seeds saved and I am going to try this spring.
Another beek friend of mine has had better luck and usually makes a bunch every spring, so I have been counting on him.
Other good ones are the American Linden, Golden rain and the weaping cherry. BUT, nothing beats the Bee Bee, it is just insane, some insects wont even leave it at night, they curl up under the leaves. The Bee Bee is a late bloomer, blooming during the dearth of July/August. Also, lack of water will shut them down, not very drought tolerant.