Does anyone know of an air powered grease gun for greasing rods may be available for purchase? Either used or a new reproduction? I know where to get a manual one, but not an air powered one.

They can be found, but they're rare. But if you find one, best be sitting down before you hear the price.
I've done a fair amount of rebuilding, repair and modification to Alemite guns, and with the cost of original units where it is, I've been toying with the idea of manufacturing a few new "copies" with improvements at a lower price.
If you need one pretty soon, contact me off line and I can send you contact info for someone who may have one for you.
Mark D.

Try; Midwest Locomotive & Machine Works.