It's mostly luck... Use your preferred search engine to find inventory, look for where it says in-transit. Email every dealer with a listing and ask if any of the in-transit inventory is available, and if not, can they keep your information and contact you in the case of a cancellation. It helps if you have a prior relationship with the dealer, they are more likely to hook up repeat customers.
Having just searched exhaustively, I can tell you that 5.3L are easy to find, even at supplier or better. As long as you will take a white one. Anything with the 6.2L or 3.0L are another story, even if you find one, you will have to pay a markup.