Mossberg sells them, but it looks like they are out of stock:
This is a discussion on Where can I buy within the Defensive Rifles & Shotgun Discussion forums, part of the Related Topics category; ghost ring sights for a mossberg 930 and a mossberg 590?...
ghost ring sights for a mossberg 930 and a mossberg 590?
Mossberg is perpetually out of stock. I doubt you will get that one from Midway, either:
“Special Order products are ordered from the manufacturer when your order is received. Once placed, Special Orders cannot be canceled or returned. All returns must go directly to the manufacturer.”
They come up now and then on eBay. That’s how I got mine for $53. Your best bet is to create a Saved Search on eBay that will email you when one comes up.
Current EDC: Glock 23
Pick up an issue of ShotGun News locally and check. Then try Brownell's.