Here is a link to the invitations that I made which had pockets inside.
The description has some details about how I made them, if you have any other questions feel free to pm me.
I have also seen threads that show how to make unique folded pocket cards that do not require taping on a separate piece of card stock to create the pocket - you can try searching for posts on "pocket folds" or "wedding invitations" to try to find those. I think those require 12X12 card stock.
To do it my way I would definitely recommend purchasing stick strip at Micheals, there it is called "Terrifically Tacky Tape" and it is lined with red, but at Michaels they carry a very narrow strip and that worked perfect for making the pockets.
Hope this helps. Like I said feel free to ask any questions, and I'll see if I can answer them for you. Good luck and have fun.
Wedding Invitations Inside by yooperbrat at Splitcoaststampers