I'm trying to figure out how to easily switch crank arm length on ultegra 8000 50/34 to something shorter (172.5 from 175).

A couple of questions:
1. I cannot purchase right arms without the rest of the set, correct?
2. If this is the case, would purchasing an entire crankset with different rings (52/36, 53/34) allow me to swap the new arms in, and give me some spare rings of different sizes to use?

Thanks!

I suppose so, although you'd have to make sure the new set used the same bolt pattern and bcd as the old one.

I supposed that's my question - I'd just be purchasing a standard ultegra 6800 52/36 or 53/39 crankset.

I knew a guy at a bike shop. At times he would sell me partial kits real cheap. IF he had a rear brake caliper he would sell me what I needed from parts collected on bikes that had been serviced in the shop.

Once I need the left crank arm on a bike where I had stripped the pedal threads. He found a match and sold me the arm for $5.

I also got a micro adjust seat post taken off a bike that swapped seat posts at time of purchase. I got it for $20, still brand new. Something that otherwise probably would have cost me $100 elsewhere.

Might be worth it to ask at your local shop. Some guys collect that stuff. Crank arms, seat posts, brake calipers etc.

I can't say for sure that you can't buy the crank without the chain rings.
Call your local shop?
Check some European bike sites (Wiggle/Ribble, Universal, Bike24, Bike-discount, et al)?

But yes, if you buy another Ultegra 6800 crankset, you will be able to swap the rings over. All Ultegra 6800 cranks use the same 110 mm, asymmetrical, 4-arm pattern.

But before you start mixing-and-matching, I bet that the ramps and pins (shifting aids) on the chainrings are designed in matched pairs.

Chainrings are usually cheapest when sold with a crank, so I'd buy complete.

Unless your LBS has one that they have taken off a bike and held on to it, I have never seen a crankset sold without the chainrings commercially. I tried looking for a Tiagra crankset for my old Defy after one of my LBS stripped the pedal threads in the drive side crank. The bike came with an FSA crankset, which I didn't like, and had the same BCD and bolt pattern as the Tiagra. Couldn't find one anywhere and ended up buying a complete, with chainrings, crankset.