Trying to source a replacement boot for the airbox due to mine being cracked/dried out but discovered the part was discontinued. It’s the elbow that goes from the airbox to the intake that has different diameters on either end. Has anyone found a suitable replacement for them? 

The part in question: 
https://www.euromotoelectrics.com/product-p/manifold386.htm

If you buy it from this seller, you get a free filter housing with it.

That part number is not discontinued, just backordered. There are 40 on order worldwide. BMW AG has them in the price book and not listed as No Longer Available. The caveat is that the wait might be a while yet. US retail price $36.90