SWEETLATINA, Is that what it really is? David's Bridal say they are backer cards. You just slide the other card in the slit. Thanks for the info but I checked there also.
I think you can definitely make this yourself. All you would need to do is cut 2 long slits and voila! I'm not sure how time consuming it would be but you would need a ruler, exacto knife, and a cutting mat to lay the cardstock on. It sure would make your life easier if DB had these in stock. I just googled and it's also known as glider cards.
Check this out: http://www.lcipaper.com/gliders.html
Also check with Target they hae a great selection of brown invitations very similar to these. I got the pocket folds from there and they came out great