The UCSB English Ballads site will give you hundreds of broadside ballads from the 17th century. ECCO will give you access to thousands of ballads that originated in the 18th century. However you can never fully guarantee origins.

If you pm me your email I can send you a copy of my earliest versions spreadsheet with about 750 songs many of which very likely originated in that period, But again I must emphasise that even this is earliest EXTANT versions.