I've bought a Leonard jersey from NBA.com with no problems. Select the custom jersey that you want (it shoud have the #00 on the front). Once you select that, there will be a drop down menu to the right that says "select a current player" when you click on that, it should give you the options of whatever player ya want. Give it a shot and let me know if it works!
I think he's going to be a star. And as time goes on, he'll be the face of the Spurs, I think.--- Coach Pop on Leonard