Regarding the low C extension, there's a basset horn and a basset clarinet. The basset clarinet is now just a standard Bb clarinet with an extension to low C. Looking at the Steve Fox website, you can even ship him your clarinet's lower joint and he'll make the extension and modifications for you. A basset horn is generally pitched in F and has a bit of a curve to it. For the Steve Fox horns, it looks like the curve is only in the barrel. For the Selmer model, it looks like your standard alto clarinet (curved "neck" and curved bell) with an extended key range.
I played bass clarinet and contrabass clarinet for a couple of years. I never saw any music that had anything below Eb. I've had a couple folks mention that the extended range is used in some avant-garde music.
If you think about it, the extended range Bb clarinets should go to low D. That's concert C.
Anyhow, the Selmer Modele 65 low Eb bass is $12,100. The Modele 67 low C bass is $14,100. The difference in price for the Buffet horns is only $1,000. Looks like Selmer's the only one building contrabass clarinets, these days, and they're low C instruments.
(I will note the basset clarinet is almost always in A)
I've played bass in bands for a while-I've seen a few parts with a low C-most recently Omar Thomas's Of Our New Day Begun, which has some low Cs and a lot of low Ds on the first page! Generally most stuff won't have a low C unless it's from after about 1985 or so, roughly.
Benedict Eppelsheim makes a Bb contra, but I wouldn't call anything of his "normal production"
Buffet also makes contra-altos!