The classifieds section of the Marlin Owners forums have sling swivel bands come up occasionally.
But the most effective choice will probably be to drill and tap the band that you have. The cheap, easy, redneck way to lock it together is "permanent" thread locker. The proper way is silver solder and refinishing.
They also have a website listed so you could probably contact them. They are a Canadian, and I've never purchased anything from them. I was unable to locate anything in the U.S.A.
I don't recommend doing business with that Canadian gun parts company if you are not in Canadia.
In my experience and that of an acquaintance, if there is ANY problem with the parts or order, they fall back on the cross-border 'protection' that they have and resort to a much more vulgar form of: "Tough cookies. We still got your money. Whatchu gonna do about it..."
They screwed me once (wrong parts). I ended up brushing it off as a mistake followed by a bad day for whomever was in the office.
Same for my acquaintance.
They screwed me harder the second time, for much more money. (Completely the wrong parts, in bad condition.) They acknowledged that the parts were not what I had paid for, but refused to do anything about it unless I shipped everything back, guaranteed overnight delivery, insured, adult signature required, ON MY DIME, and in violation of export law (~$135 at the time -- not to be refunded by them).
Due to some Canadian financial law, the CC company couldn't even do a charge-back. We spent months in arbitration, which was eventually settled 100% in my favor. But they still couldn't force a refund or return. It was merely "suggested" that the company do the right thing. They refused, but sent some childish, vulgar celebratory taunts to myself, the CC company, and the arbiter.