I didn't get a response back from Messicks yet, but Golden Eagle and local dealer both tried to order but were told it was discontinued.
Kubota customer service replied "Thank you for your inquiry to Kubota. The parts you reference [15585-11620] are no longer in current Kubota inventory. You may wish to contact a company that has rebuilt and / or salvage Kubota parts"
It seems like the air inlet pipe (and airbox) used on the L275 is different from any other model - I looked through the parts manuals for just about every L series made in the 80s and none used the same parts as the L275. Maybe someone knows if one from a different model can be cut down to work?