Articulated C# key clarinets
You have one sure option for this, and a few "if you are lucky" ones:
Your One "Can't Miss" Option:
The only maker to consistently offer an articulated C# key is Amati. They offer it on their "full Boehm" clarinet, for which you will expect to pay a premium if you decide to buy one. Woodwind and Brasswind, the on-line everything music operation will get you one in a week or so, for a little more than you would expect to pay for a basic professional clarinet from any of the French or Japanese makers.
Be prepared to accept less than optimal finish on the instrument. My Oehler "professional" horn from the same maker is not up to French standards of fit and finish. You may see any of the following, all of which can be fixed but a price to you:
1) Tone holes with rough edges, caused by dull fly cutters
2) Rough finish on the back of keywork (cosmetic, but still)
3) Poor workmanship on key work (I had a stopper post (under a long key on the bottom joint) that fell out when the key on top of it was removed; thank God that Fred (my chosen repair guy in the Houston area) wears a "dropped screw" apron when he works, or we would have had one hell of a time getting a replacement part)
4) Poorly adjusted key work (my rings were set WAY too high for my fingers to seal the holes)
5) Poorly set pads.
The maker's marks on the front of the horn are also butt ugly...
Your Other Options (Which Are Not So Certain):
The other options are all either completely custom (Orsi will build you anything if you have the bails of lira to spend) or aftermarket:
1) My favorite, and the most attainable by a long shot, is to shop eBay for a "no cracks" Selmer Series 9 horn. You can usually find these in one of these forms:
A) Boehm system with the articulated G# key, and fork Eb/Bb mechanism
B) Boehm system with the articulated G# key, fork Eb/Bb mechanism and low Eb key
C) "Full" Boehm system with the articulated G# key, fork Eb/Bb mechanism, LH Eb lever and low Eb key
Remarkably, one seldom sees a horn with just the articulated G# key, fork Eb/Bb mechanism and the LH Eb lever. I don't know why this is; they certainly had them in stock when I picked out my instrument, and my (used and rebuilt) Series 10 is so configured. But, they just don't show up.
The "Don't Say I Didn't Tell You About This One" Option:
You can also find Selmer metal clarinets with the above variations of key work. Despite popular opinion that "all metal clarinets are junk", these Selmer horns are anything but. I have one, sitting in the closet and as of yet unrefurbished - the sliver plating is the main obstacle to a complete overhaul.
If you wait around a while, perhaps Chris P will post a knuckle-bitingly beautiful photos of his two horns of this ilk.
( Every time I see that photo, I hate that guy a little more. Envy can do that to you...)
Your "We're Running Out Of Options Here" Options:
You will also occasionally find other Selmer clarinets so equipped. If your tastes run to Buffet instruments, a Series 10 or 10S instrument so configured will occasionally surface.
Also seen are earlier Selmer models like the Centered Tone that are provided with this key work. However, the older horns are often banded (to repair cracks) or cracked, and are not provided with such modern improvements as the poly cylindrical bores.
And Then There's Leblanc:
Run by a differently thinking bunch over in France, Leblanc Paris used to be the last refuge of the full "Boehm" clarinets. At one time, their catalog was a smorgasbord of special key work instruments, horns that you could order off of the rack. Sadly, this is no longer the case, and (despite a breathtaking assortment once seen in the catalog) such instruments seldom appear on the secondary market.
I do a sweep of eBay every other month or so, just to see what can be had, and (in years of doing this) I have seen only one Leblanc instrument with "auxiliary" key work, that being a Boehm system with the articulated G# key and fork Eb/Bb mechanism. The firm used to offer a Pete Fountain model with gold plated key work and everything but the low Eb key, but those have disappeared from both the catalog and the secondary market, as if dropped in a bottomless pit.
Slim To No Chance, Bottom Of The Barrel, Options:
You will (VERY) occasionally see a Buffet instrument with variations on the full Boehm setup. Occasionally means I've seen maybe two of these over the years.
As for Boosey-Hawkes, I have never seen such an instrument. I have seen precious few B&H horns of any stripe, much less an up-end horn with the extra key work.
And, Heckel once made them. I've never seen a Heckel full "Boehm" horn, this in forty years of looking.
So, there are your options. If I was to bet on your success, I'd put my money on a decent Series 9, purchased off of eBay and rebuilt by a competent repair person. But, then again, I'm biased in that direction...
Just out of curiosity (and as a service to you, the clarinet user and consumer), I'm going to do an "eBay census" right now, just to see what your chances are.