NASA occasionally releases new poster sets that you can download and print for free.

The annual OSIRIS-REx commemorative poster, featuring Earth Gravity Assist.

Mars Explorers Wanted Posters. There are several different Mars posters in this set.

Orion Boldly Going Beyond. This is a single poster.

Visions of the Future is a set of travel posters to places like Ceres, Titan, Enceladus, Mars, Jupiter and beyond. There are several posters in the set.