The clarinet family is badly named, in a couple places. There is an Eb soprano ("sopranino") clarinet, Eb alto clarinet, and an Eb contralto clarinet. The contralto is sometimes referred to as an "Eb contrabass clarinet." The Bb clarinet line consists of Bb soprano, Bb bass, Bb contrabass, and Bb octobass -- and there's only ever been one octobass built. That's by Leblanc.
Just as a recap ...
* There are no modern C bass clarinets.
* There are a couple places that could custom make you a C bass clarinet if you have at least $20K you're willing to spend. Possibly more.
* If you have a written piece of music and you want a LIVE bass clarinet player to play it, either transpose the part to the correct key by using Sibelius or another notation software OR make sure your bass clarinet player can transpose on sight.
* If you have a written piece of music and you want to use a bass clarinet patch in your DAW or sampler, there's no need to transpose.
If you need a bass clarinet virtual instrument, this one sounds phenomenal and it's only 50 Euro for the Standard library and 90 Euro for the standard library and some extra expressions. Arguably, you could go with the Audio Modeling clarinets pack (higher price), but the bass clarinet doesn't have much growl on the lower end.
Okay. If I WAS to look for a custom C bass clarinet, what are the places?