I've never noticed it for sale flower, but you could always call the mill. No, I'm serious, call a lumber mill (or lumber yard maybe?)
Careful though - some pine needles kill off other plants (hence the sparse groundcover in some pine forests), so who knows if the sawdust will do the same to your grass.
Obviously, don't get dust from MDF or engineered wood since it's bound to be poisonous for man and beast.