Speaking as a diver:You wont typically find 300bar tanks at dive shops...because most divers dont use them due to the weight, fact.
Also fact I believe, is that 12L 300bar tanks are usually made of steel, not aluminium.
It is not adviseable to retro fit an A-Clamp fitting onto a 300bar DIN tank, as A-Clamps are commonly only rated for 207-232 bar...not 300bar.
Dive shops sell loads of 300bar tanks, many divers use them due to the increased dive time they give and most tanks are made of steel. Hence the tag "If it's steel it must be Faber"
see www.divebox.co.uk
Aluminium tanks tend to be lower capacity (3 to 7 litre) used by rebreather divers and as stage bottles.
a-clamp on 300bar is a no-no