...yeah, great ask!
i used Lincare (WA) for many years ....they initially did not carry M tanks and were none too interested in doing so (my guess that the cost of inventory, storage space, supply of tanks/regs, training, and just plain ignorance of need all potential reasons...they never would say). But, i asked so many times that the need was known, and one time they surprised on arrival at the O2 shop with a couple just for me....and the tech "loaned" me the proper regulator. Took a while, no guarantees ....but for the sake of clusterheads down the line, asking for M's can help spur motivation to supply. Econ 101 (the last one i ever took) "supply evolves to meet demand"....