Best place to buy MH stuff is from MH in the USA who sell direct.
I have just bought one of their carbon fibre wrapped bottles, a big fat mama at 48 cu ft @ 150 bar, for USD 650.
This bottle comes with a 540 valve and a pressure gauge on the valve, so you can see how much gas is inside without having to screw the first stage regulator on it (very handy).
I've done extensive comparison tests of o2 systems, at FL180 so far but with a FL120 one planned, and indeed the MH system comes out tops on gas usage, by a factor of about 3x over the other demand regulator on the market (Nelson precise Flow). SR - you have email. However, there are certain downsides to the MH system: the requirement for electrical power
for the demand reg (it fails SHUT) and the rather abrupt pressure pulses which may irritate some noses (though I am sure most GA noses have plenty large enough apertures from years of picking up and spreading airfield gossip).
As regards FL140 without o2, I am sure this will work for some people for an hour or two but I am sure most people will soon be unable to do anything useful. I have flown with a pilot who could not read the altimeter after an hour at FL120 - he was fine within seconds of going on o2.
Pilots who fly these trips without o2 are just chucking away their best weather avoidance option: VMC on top. At the best of times, they arrive very tired...