Star Instruments does the R-C type of cass in a number of sizes. I don`t know whether it`s a side business or a hobby business but the guy has done some rather large optics on occasion. Other than that I am unsure of makers. Cassegrains tend to be custom instruments (other than the SCTs that Meade, Celestron and others make). I might note that the 16" SCT of Meade`s is usually a very nice instrument and did hear of one that a observatory bought for some silly reason and never set up. I I had been really smart, I could probably have bought it for a song.
The other thing is to build one yourself or with the glasspusher guru in your area.
Good Luck.