I am looking for a domestic source for unterminated masterlink cabling, junction boxes, etc. in order to wire 2 BL 3500's.

Any B&O dealer ought to be able to help out.  

Sounds Heanenly click the on bottom of this page this guy is super nice and reasonable free shipping and fast talk to Steve

Yeah, I figured that, Russ!

With B & O moving to Cat 7 wiring, I would think there should be no issue getting that wiring on my own.  Does B&O offer end connectors only, or do I need to purchase a finished wire and cut off the dog tail (which is ludicrous to me, honestly)

As ludicrous as it sounds....  B&O offer an ML cable with only one terminated end for applications like yours.  Order those, the appropriate number of punch-down blocks and the bulk cable, or a finished cable in the length you need.  

As for complete DiY...the crimping yoo will set you back more than $900USD.

Russ

As for Cat 7, you probably ought to do it that way and only order the wall plates ans pig-tails from one of us.  At least you'd be a step ahead then.

Russ

I have access to a crimping tool, etc.  Once I map out how it's going to be wired, I'll roll from there.

In a search, I did find Bosscom has some perfect add-ons (including an outdoor mount for IR eye): yet, they are a wholesaler/supplier: anyone use one of their retailers n the past??