My favorite local spot is Champa nursery in El Monte, but that is in Los Angeles county. Prices are the best I've seen so far in the Los Angeles area for Florida grown trees. Btw, many of the Orange County CA nurseries get their trees from Champa. Champa will probably have the most selection.
It's best to call to see what varieties they currently have in stock. Jimmy, the owner, is a really nice guy.
In past visits, Champa has carried NDM#4 and Kesar. NDM has a florally scent/flavor while Kesar has a strong Alphonso-like piney taste. It's too bad we CA folks don't have mango fruit stands like Florida has so that we can try different variety fruits before buying trees.
To answer your original question, some Orange County CA nurseries that sells mango trees are:
Laguna Hills nursery, Green Mart, Mimosa Anaheim, Flowerdales, and Emily Garden.