They can be ordered. When we Frankenbuilt my Expert Di2 off a Comp frame, it took 2 months for the kit to arrive.
By the time it arrived, I was convinced I didn't need it, and I loved the flexibility of mounting cages lower than usual... and then I saw it, filled it up, used it for a couple of rides, and now I'm not sure how to convince myself to ride something without it. It's like a saddlebag that you don't have to dig through, and that doesn't waddle 'neath yer nethers when climbing. The new Diverge has mounts for it also....