Buy the smaller trees. They are easier to transplant especially at the start of summer. If you provide lots of water it will grow quickly. The key for most desert plants is the amount of water. The more water, the faster and bigger it will grow.

Hi Jake
You didn't say where you are at in S Calif. Palo Verdes can be found at most local nurseries in the Palm Springs area. They are used there a lot. The Living Desert may also have some for sale.