I agree, I wish there was a better one to last forever.
My green o-ring that first came from Mossberg was in pieces after being on the shotgun for 20-years. I never shot it, but time and age destroyed the green o-ring if you ask me.
Last month got a package of 10 black O-ring about the same size from the Home Depot plumbing deptartment for $2.00 (very cheap substitute). I have only put 3 boxes of target grade #8 shot with the first replacement plumbing black 0-ring so far and no issues. It still looks like new.
I did order one of the O-ring ($2 each) and they were made as a replacement by Havlin Sales below (color is brown). She was very nice and great to ask questions. http://havlinsales.com/