I mainly use bit key blanks, and large sizes are not so commonly stocked. As well as some lever locks, they are also needed for warded door and safe locks.
Two particular blanks are Cotterill Climax, and Cotterill-Wilson blanks.

I have made Yale feather keys from flat steel blanks, no problem. But I don't know if Yale corrugated blanks are still available (not that I need any myself).