Its a thicker piece of paper that is stronger than normal paper but not quite cardboard, its the same stuff that buisness cards, playing cards, etc. are made out of hence "cardstock"
Construction paper is very similar in use and usability, you can probably get it at your local arts and crafts store, if not [link|https://www.amazon.com/Neenah-Cardstock-Heavyweight-Brightness-SHEETS/dp/B07QQ3L753?source=ps-sl-shoppingads-lpcontext&ref_=fplfs&smid=ATVPDKIKX0DER&th=1|here|new_window=true] is a 300 piece 8.5 by 11 inch box on amazon