I have seen just the Muhle head for sale at The English Shaving Company, though it is not open comb. CoonCatBob, a member here, might have some vintage Gillette New razor heads for sale. A WTB ad in the B/S/T forum would probably get you one as well.
Or you could just buy an OC razor and then screw the head off of it. That seems like it might be the easiest solution. You can get a NEW for not that much coin.
There are many, many vintage Gillette open comb razors with unusuable handles that have split. These can be bought for pocket change. Flea bay is your friend.