For one time use templates, I just use poster board from staples...
[URL unfurl="true"]https://www.staples.com/Staples-Poster-Boards-10-Pack-White-22-x-28-MMK04500S/product_247403[/URL]
For something I'll use multiple times, chipboard is more durable;
[URL unfurl="true"]https://www.staples.com/26-x-38-staples-heavy-duty-chipboard-pad-70-case/product_24406652[/URL]
But you could probably find that cheaper (Smaller Quanity) at someplace like Michael's or the UPS store, it's commonly used for stiffening large envelopes, or putting in the top of a box for shipping as extra protection.
