The cheapest Ive found the Marinco kit is at Amazon.com. Here's the link:
http://www.amazon.com/ParkPower-Mari...+30+amp+rv+kit
Amazon also has an "off brand" that supposedly is just as good as Marinco, but I can't attest to the quality. I went with Marinco because I know what quality it is. Anyway, here's the Connnetk kit. It's about $40 cheaper than Marinco.
http://www.amazon.com/Conntek-RV30CV...+30+amp+rv+kit
Now, if when you start trying to install the kit and the adapter plate won't cover your entire present cutout for the mousehole, don't panic. Simply go to WalMart, buy a white plastic cutting board and fabricate your own plate to install over the mousehole.
I think either kit will work well for most RV use, as I said, I went with Marinco because I've always had good luck with their products and I know the quality is good.