Thanks for the tip. Just bought one.
https://smile.amazon.com/DGK-Color-Tools-inch-Digital/dp/B00HP8JSU8/ref=sr_1_15?crid=1E4VQO2O4ODDL&keywords=18%25+gray+card+photography&qid=1583848563&sprefix=18%25%2Caps%2C148&sr=8-15
