Links to Do it Yourself Cardboard Kits
Here are some links to articles which explain how to make your own Google Cardboard kits using old cardboard boxes. You'll still need to get the lenses and NFC tags (an optional accessory).
The files you'll need to make your own Cardboard
When you print these you'll want to print them tiled at 400% so they come out the right size then glue them to the cardboard so you have the pattern you want to cut out.
Some of these are optional, but I wanted to make sure everything was available for the people who wanted to go all out.