Couple of sites for you. Some states have published guides to their state archives and historical library that can be very helpful. County boundary changes will be found here because they involved legislative action. Other useful records that might be found here are land grants, early marriage records, newspapers, militia muster and pay rolls, pensions, immigration and naturalization records, tax lists, biographical materials, and, of course, census returns. Some states have programs of gathering county records into the state archives.
Some state archives have material available on states other than their own.
Check with the state historical society of your state of interest to learn what records they have available for researchers. Some states took a census in the years between the Federal census. These returns can be helpful in tracking a family between the Federal census years. Some states have returns that extend into the 1920s. There you will find records from 1775.