check the https://cantusdatabase.org database. Search for a well known (likely illuminated) chant from a holy day and see what pops up. Some archives permit free usage, others don't. It depends on their individual copyright terms, but with a little homework, I'm positive you'll find something nice here. Considering you would just be using it for a poster, and not for profit, you could probably use just about any graphic under "fair use".