Some people on here use antlers for their dogs to chew, deer or elk. I've used raw marrow or knuckle bones but my older boy chipped or wore down all 4 of his canines, so those bones had to go. Not all dogs are wild about antlers, my little one used to chew them, now not so much. My sister's dogs went wild with them when I gave them one Chase didn't want. You could try one or 2 for them and see what they think before buying a whole bunch. I couldn't say if they're harder or less hard than Nylabones but they seem popular. Maybe others will give their experiences with antlers. Here's one place I've gotten some before:
Grateful Shed Antler Chews for Dogs, Wholesale to the Public