New scope problems with cantilevered slug barrel

[Dan]

Hello all. I've been an 870 owner since the 70's and love them and have a fee. I'm upgrading my original Burris Scope to a newer Leupold and have found that the old mounting rings do not match up. That's ok if I could find the same diameter rings as the scope, but I cannot find any. Any websites that you could suggest for specific rings that would fit the cantilevered would be awesome! Love these guns. Thanks.