This is what you want.
https://www.gastronomicspain.com/blog/en/acorn-fed-iberico-pork/
One of the most commonly found trees in Spain is the encina, the holm oak. This is not the same as the roble, the oak tree that you will be familiar with. The encina is evergreen, but like the roble also bears acorns. The best ham comes from pigs that roam free-range over the ground/grass under the trees, and eat the fallen acorns.
As you'll see from the article, what you want to look for is jamón ibérico puro bellota, and if you really want to give your wallet a real workout, then add pata negra.
In the U.S., if you're in the Dartzeel bracket, you can shop here while visiting Colonial Williamsburg:
https://www.tienda.com/jamon/bone-in-hams.html