12 x 12 cardstock?

Place it under a book.

Hi. Use acid free tissue paper between each and store in a drawer or closet. You can also use an old picture album and put into.

How about a firm box cut it down and store it in there

Or troffice Depot or Staples they will have things for offices to organize

These plastic cases are designed for this size of card stock storage. Inside the case, the paper is kept clean, and is protected from becoming warped or dogeared. The case can be stored flat, and is strong enough to withstand some weight on top. If the card stock being exposed to either dust or bright light are not concerns, then semi-open style racks are also available.

Thank you.

They sell 12x12 cardstock storage boxes at most craft stores.