Actually, I don't think BX exists any longer. You are looking for type MC (metal clad) which comes in both steel and aluminum jacket. The aluminum is much easier to work with, the steel is more protective from crushing forces. There is also a limited bend radius when using this. You can finf type MC and the proper fitting at HD
Sorry -- it's a trap that even I, code-weenie that I am, fall into. I was using the older term BX to refer generically to both MC and AC types of cable (metal-clad and armored). The principle difference between those is the use of the cladding for safety ground on AC (and the old-style BX) vs. an included ground conductor in MC. For this reason, MC is preferable, IMO.
And, yes, fittings for both are available wherever the cable is sold. The package will say right on it what cable it is approved to work with.
If I may suggest, use the 24' one for now. Get the longer one later. Not that hard to switch them later.
I agree with this 100%. Especially if you go ahead and use a "separable" cord arrangement such as with the Marinco inlet we discussed. We're only talking about a foot difference, and the older code only called for 23' anyway.