Hi
Indeed large sizes are very difficult to find. I searched for months for a BMB in 7 1/4 or 7 1/2 but only found 6 3/4 or 7 on ebay or in military surplus. I asked SOF, they don't sell reproductions anymore, probably because DR helmets are not enough interesting for them, the DR public beeing too small. At last I found a 1942 BMB 7 1/2 but for 250 euros (average price for smaller sizes: 150 euros) at a surplus store in Normandy. They know these sizes are rare, so prices rise...
Maybe you can find more easily a belgian post-war one, and at more decent prices (they are exactly the same as the BMB's)
Checkpoint militaria has some coming in regularly