If the Ruger gods smile at you, you may still find a set of factory walnut grips floating around at a gun show, but if you do find a set, you will pay and pay, dearly, for them. The odds are very much against you on getting factory original grips, but it never hurts to check.
Also, be aware that there were two grip frames used on the Mark I - the earlier A54 and the later A100 and,yes,they take different grips. The change to the A100 frame was made sometime in 1972
Custom is the most likely option, but not many custom makers even list grips for the Mark I, anymore. You might try Tombstone, bit if you want walnut, you'll most likely have to visit with a custom maker.