I saw a couple of tremoloas available on ebay recently. They weren't as expensive as I expected but then again, there's probably not a huge demand for them. Hadn't heard of the other two but I looked them up and they seem like cool instruments. I've never played a bowed instrument before though.
They sound a bit like bowed psaltry, only a little fuller. One thing to keep in mind is, at least with the older ones, they are very difficult to tune and to keep in tune. Helps to have perfect or relative pitch, too.
That's good to know. Bowed psalteries seem like they would be fun to play. I think it would be really cool to add a pickup and play one through an amp.