Many of the sellers are selling acres of desert land and have no interest in subdividing their land into small parcels for billboards. However, there is a zoning called "Highway Commercial" which many of the big billboard companies' boards seem to reside, I don't know if this land is owned only by private parties or perhaps by the State/County. Govt agencies aren't too receptive to give that info.
I think you have to have strong reasons to change a zone from resource conservation (aka "public lands") and have a lot of patience & a good amt of funds, you can pull this off. Just an FYI, nearby railroad tracks or fiber optic lines don't qualify as a neighboring business. I already tried that one.
Unfortunately, the billboard business doesn't come with easy-to-follow instructions to become a success, and the CA regulations which have been passed in the last 10-15 years makes things even more difficult but not impossible. Only the very persistent and tough survive in this field.
Good luck, Garrett. Feel free to contact me offline if you have more questions.