Just the first day of the Shot Show so we should be getting some news. I am just hoping that HK is giving out some free swag that I can pick up on eBay.
HKParts is usually (not always) the most expensive place. I ordered a handful of green MK23 barrel orings and a few slide parts as they had them when others didn't but I always do research first.
Charlie's seems to be the only place that has ever sold them commercially. If you search the part number or UPC, it's all CCCs results. Check with https://mfiap.com/ I know that they have stocked quite a bit of 551-A1 parts.