Commercial two way radio, as we have always known it, is all but dead! Along with that, slide mounts. You might try here: http://kollmanradio.com/
They used to carry the Wilson slide mount line, and they might know what is still available, as they cater to the amateur, and the CB markets.
By far the best one, was made by Motorola. It was designed for their Micor-10 series, but adaptable to just about any radio. The mount even had a plug in coax connection, good to at least 800 MHz. Occasionally, you see one of these at a hamfest.
Good luck on your search.