I have seen late model 150mm Ritchey stems around...maybe through their custom department.
In my stem stach...I have an old school Bonty 150mm Racing stem that I have ridden on my 29er...designed for old school mtb XC. On a shorter stem now tho. Have ridden 140mm on my road bike. I don't see stem length as a big deal. Pro's ride smaller bikes with long stems for the shorter head tube to get more aero.