I am not sure what the exact mount you are looking for, but check out the Ham radio outlet. ( Hamradio . com ) They also have quite a few stores throughout the US, and there may be one near you. If you can go they have a wall full of multiple types of Antennas, mounts with the various connectors needed.
I'm not sure if you want CB or Ham, but most of the CB stuff I've seen for the big rigs share similiar to some Ham antenna mounts. Also, check out your nearest Pilot of Loves truck stop store, they have a good selection as well, however they mostly deal with CB.. Good luck..