I know I’m a few months late on this, but maybe check with local photographers, especially ones who would turn over a high number of prints…
My dad owns a studio and does a lot of portraiture and sports photos so he has a constant stream of prints coming in from his lab. When he picks up the prints, they come sandwiched between chipboard to keep them from getting banged up. He used to just give the chipboard back to the lab to reuse but has been keeping about a ream’s worth on hand for me for whenever I need it.