Boonie,
A ball-mount with an SO-239 connection isn't the most common type on the market, not 'rare', but not common. They are available though, so it's just a matter of finding a seller. The 'stainless' part may be a problem too, I honestly don't know. The most common place to find a ball-mount used to be Radio Shack. Having used both their very old ones, and the newer ones, I would not recommend the newer ones unless you are desperate. Keep looking, you -can- find them.
There are alternatives. Google 'Breedlove' for antenna mounts. They won't be stainless, but are brass. They are also not the cheapest things on the market, that's referring to both cost and -quality-! If brass will work for you, and if you don't see what you want, this guy will make any thing you want. He's a very good machinist.
Not much help, huh? It is an alternative though.
- 'Doc