I second the CountyComm recommendation. They've got 2 sizes. Also there are some pistol mag holders that are horizontal. Blackhawk makes some pouches that might fit your needs.
I bought his horizontal rig via Police Bookshelf or was it LFI? Anyhou, it was the best way I have ever carried the 6P and/or 9P. Besides it slides along the belt. Used it for a number of years until it finally wore out.
Glad I saw this thread! Now I know where I can get some. . .