build it yourself.. with the help u need. It's not very difficult
I believe that the shield u alredy got, also can be used on a mega..
The extra I/O can e.g connect (via cable)
to mega pins 22..54,using Cable & Wire Connectors for sale | eBay
in both ends..
and
http://www.ebay.com/itm/10Pcs-DIY-Prototype-Paper-PCB-Universal-Experiment-Matrix-Circuit-Board-5x7cm-/180817332600?pt=LH_DefaultDomain_0&hash=item2a198d8978
..some pins must be reserved GND
Your 'private' shield can, of course, be a plug-in type..