Use Broadside* as a keyword term, limit to the period of interest, and add extra keywords as needed.
In worldcat, because there is so much material, search Broadsides as a subject and limit by publication date, adding extra keyword and subjects.
If you find a broadside of interest in WorldCat, search online to see if it is digitalized. If not, go to the website of the library that holds the item and see if ILL is an option.