Heidi, chair of CCDP, has created and shared free digital downloads for post cards.
Check ’em out! There are two versions.
That page has instructions. Like, the digital download is intended to fit well using Avery Product #8387. Easy to find at office supply stores and “big box” stores.
https://www.avery.com/products/cards/8387
As of 30 minutes before posting this, Top Office in Sheridan has a few hundred of the blanks for this size card, although they are a different, discontinued, product. And can order #8387.
Cheers!