Boy, I sure hope you are able to get some opinions on this.
Just kidding.
I like the Limbsaver series. They are real soft and work as advertised.
A buddy of mine likes Pachmayer Decelarator, as do a number of other folks on this board.
Both are available from Brownells, Midway, or dozens of others and either would be a good choice.
You should have no trouble finding a good pre-fit for a Mossberg 500.
With all that said, painful recoil is usually the result of bad fit, or bad technique.
Not saying that is the issue for you, but more often than not it is the case. Lots of us are more sensitive to recoil than others. I guess I am kind of an insensitive lummox when it comes to recoil. It just doesn't seem to bother me that much, although I do have Limbsavers on 2 of my shotguns. The over / under that I have been shooting lately just has a plain 1/4" rubber pad, but doesn't bother me at all. I usually fire 100 or so rounds through it on the weekends.
Just thinking out loud here. Maybe it is conditioning as much as anything else. When you regularly shoot a lot (I shoot most weekends), maybe you just become used to it. The shoulder and pectoral muscles get conditioned and hardened so that it doesn't bother you anymore.
Yeah, that's the ticket...just shoot more.