I've had a lot of success with cuttings of Crape myrtles. I usually get a 4-6 inch cutting, dab the cut end with rooting hormone and place in moist medium. They root very easily!

Air layering is not the best way to propagate anything that will start easily from cuttings. Al

You could also just cut a root as they run close to the surface. Where you cut clear some of the earth away. A shoot should come up from there which you can dig up and transplant later. You may even get a few come up on their own, but not attached to the tree its self, those you can start as dvtown has mentioned. You should be able to start all the starts you like within a few years to where you will have to give them away.
Paul

When to take cuttings and how long it takes to root?
Arif