I found some great patterns for quilt block cards on the Bow Valley Calligraphy Guild website. Lynn Slevinsky, the contributor of the project, is a talented lettering artist who publishes some great books on easy marker lettering.
Edited to add: I started with white piece of cardstock 3" square. I cut patterned paper 1 1/2" square and then cut those into triangles. This is a standard 1/2 sheet of cardstock card.
Even with a pattern, this took longer than I thought it would, but I love the finished result. They're a bit fiddly but I just might try another!