Information about ordering reed plates for all the Yamahas is in Troy’s thread om Yamaha P-37D Reed Plates under Melodica Construction, Repair, and Maintenance. Unfortunately the cost (USA) is $25 + shipping for each plate. It’s very bad news, though, if both those B flat reeds have been destroyed by how you want to play, Lowboy.
I was just doing some Googling about harmonica reeds and found the following, which explains why reeds fail and also comments about possible implications for your style of playing. He gives Junior Wells as an example of a harmonica player “who seems to be playing loud but actually doesn’t.” If you tune the reed and then avoid playing it really loud for a while, you should find out whether or not it has failed. If it goes flat again, you’ll know it has failed; if not, you’ll know it has just been flattened by hard playing. My flat G reed eventually snapped when I was fiddling with it. It broke at the base. Maybe it’s a change at the base of the reed, where it connects, that has made your note flat. Maybe it has bent slightly up or slightly down. If you tune it and keep doing what caused that change, it will go flat again and so on until it eventually snaps.