Check out Merlin cycles. They have the best prices on shimano and actually have sales on the Di2 stuff. Usually their groups come with a crank but you could always sell it. They rival Ali express in price and for me in the US at least the shipping is like 2 days vs a a few weeks. Plus it is all legit and covered by warranty since it is an actual shop selling it.
Otherwise just search Ali express ad piece together what you want. Most sellers tend to have and then not have stock so I have to source from different ones each time I am looking.
Yeah good point. Been waiting 4 months for Merlin to get 165mm cranks. So I guess I could just suck it up and sell the 170s when they arrive