I used a generic Limbsaver slip on pad from Walmart on my M48a. It there was a size associated with it, I don't know what it was. It's been years since I bought it.
Thing kicked like a mule, with Turk surplus ammo.
Never put one on a Mosin-Nagant, but I assume the same generic pad would work well enough on them, too. It was a tight fit on the M48a and would probably be a tight fit on a MN, too. It works well, though.