They probably purchased artwork in bulk from a supplier, who probably bought the artwork from freelancers, who probably who took it without permission. Do you have any idea where the supplier or freelancers would have taken it from, was it available online in high enough resolution to print?

And now that this has been discovered, have you registered it with copyright.gov (allowing for statutory penalties, presuming this stays US-based)?

A stock agency wouldn’t take them (probably) because of chain of title, i.e., establishing who actually owns the copyright. Did your late husband have any siblings, do you have any kids, and what state did all of this happen in, etc. You’d need signify from everyone and even then someone might claim something.

Creative Commons might be an option. Or museum, historical society.