Should have just bought a model with ghost rings right off the bat, if you really want GRs. It's almost always cheaper to buy them on the gun than to add them to a shotgun that doesn't come with them, and simpler too.
http://www.mossberg.com/product/shot...r-9-shot/51772 for example...
Another suggestion - if you haven't really made up your mind on something, it usually works out better to ask here first and buy later, rather than to buy first and then ask. Sorry...