After a long day of cycling, I find my knees feel better when I do things like walk up stairs, etc., if I ride my bike with 170mm cranks than with 175mm cranks. A small change might be enough?

If you expect this knee situation to be permanent, that might be a good solution. But, if you did that, you would have to decide how short, and would you use that length on both sides?

Once you get outside the normal range of 165 to 175 cranks, the prices go up.

Shipping from the UK can be expensive, but this might be an option if you have a square taper crank. VAT would not be charged if shipped to USA. But this could put your feet out in a wider stance which might not work so good if you have knee flexibility issues to start with.
https://www.sjscycles.co.uk/componen...eners/?geoc=US