For a long time I've been searching for thick index cards to use in my art work. Office supply stores and big box stores only offer flimsy index cards that do not work well with art supplies.
I recently found a small business man (Chris Norstrom) who sells Super Thick Index Cards on Amazon. He graciously sent me free samples (above) so that I can choose the size and thickness of cards that I needed. I just placed an order for 4" x 6" Super Thick (14 point - 100# weight) cards which I plan to use with pen & ink and watercolor paints.
I have no association with Norstrom. I'm just pleased that he offers such a wonderful product.