I keep thinking that Leupold no longer makes the 550 QD mounts...don't see them listed on their website any longer, and have heard talk they were discontinued. You might be able to find them, but they'd probably be a private party purchase.
Another manufacturer you could look at would be Warne.
I just bought a set of Talley lever QDs for my 550. Consensus here, and other places, is that Talley would be the first choice.