There are several places to print posters including on-campus resources like Lamont Media Lab and Harvard University Mail Services in addition off-campus stores such as Gnomon Copy and Print and FedEx Office.
How to Design A Poster
This page contains a broad variety of templates (via programs like Powerpoint and Illustrator) and advice about how to best construct posters.
Overleaf Poster Templates: https://www.overleaf.com/gallery/tagged/poster
Better Poster LaTeX Template: https://github.com/rafaelbailo/betterposter-latex-template
How to Prepare a Scientific Poster: https://www.science.org/content/article/how-prepare-scientific-poster
Below are examples of posters produced by other students in the department using LaTeX, as well as a generic template for PowerPoint, and a helpful tutorial on good poster design.