I hate giving these half informed answers, but since you don't have a good answer in 8 days I'll tell you what I got.
The ones I've seen are custom. Somebody found a machinist to make them one.
www.practicalmachinist.com/Here is a pretty active home machine shop forum. You could try a classified ad in there but prepare yourself for a hefty price tag. Also be prepared to send detailed sizes and an drawing.
The reason I say my answer is half informed is IF there is a production one being made it would be more cost effective, but I don't know of any.