Who makes / sells these bbl's ? Do they come with the bbl extention installed, and are they a direct fit to the ar receiver ?

I don't know anyone making the barrel alone.  But the people who make the Beawulf .50 upper also make the Ghangus .21 (the 5.45x39) upper.  IIRC the companies name is Alexander Arms.

alexander arms makes the .21 Genghis in 5.45x39.  they use a standard upper receiver, but the bolt is NOT the same as a .223/5.56 AR.  as far as i know you need to buy the complete upper (or rifle) to shoot this caliber.

With all due respect, why the hell would you want to replace the .223 in an AR with a lower velocity, less accurate .22 centerfire round, in which you'll have trouble getting more than 5 rounds to load and feed in a magazine?

7.62x39 I can well understand; for deer hunting, or just to shoot bigger, heavier bullets.  But 5.45?????

the 5.45 has a flatter trajectory, and relies on tumbling ("yawing") rapidly in tissue rather than fragmentation (which is limited to well under 200 yards in a 20" .223).  although it does take some testing to find mags that work well, i usually load 20 rounds in a 30 round USGI (i have loaded up to 28 but prefer to just keep it simple: 1 box per mag).
ammo is a lot cheaper, which is why i got one (just to go out shooting inexpensively).  actually the ammo is the same price as Wolf .223, but people don't give you a hard time for shooting wolf in a gun designed for its steel cases.
i don't think it is necessarily a better round overall than the .223, but saying "why replace the .223" is like asking Glock owners why they have a different caliber than yours...or why a 9mm AR?  
as far as less accurate i don't think that's 100% correct.  last time i shot mine at 300 yards i was hitting the steel with only the detachable irons...of course, i'm not trying for subMOA groups here, just being able to hit a torso-sized target at reasonable ranges. and the 5.45 isn't set up as a bull-barrel match gun...its an M4 profile 16".
(PS-i'm not a very good shot either!)

different strokes for different folks, right?