repost from BHB - tissue for DIY invites?????

I am DIY-ing my invites, and I would like to have tissue in them, but I am at a loss on how to do it. The invites are 4x9 (envelopment pocketfolds).

Do I buy tissue for wrapping gifts and cut it to size? That doesn't seem right, but I can't find any alternatives online.

THANKS!!!!!!!