National and statewide resources that can be searched and accessed online.
Many universities have archives that deal with the history of the university and surrounding town. Collections at bigger universities are sometimes more expansive. Similarly, some counties and towns have small archives and historical societies. Many small archives have not been able to digitize their records and materials, which means those items can only be accessed by physically visiting the location, though some do have some digital content available. If you live nearby, or if you know the piece of history you're looking for is local to one of these areas, they might be worth a try. Make sure to check their websites for current pandemic opening hours and policies (e.g. many are reservation-only right now).