As with other US data which is collected, the CPS (Current Population Survey) is the best source for national statistics reported annually covering a long time period.
If you want to get below the state level, the American Community Survey is going to be your source, but there are wrinkles to comparison over time and timeliness of data, especially for smaller geographies.*
Since you said you want to go below states, and since I know the ACS best, I'll go into some more detail:
The ACS provides a number of tabulations about income distribution quintiles. After each, I've linked to Census Reporter (CR, my project), Social Explorer (SE) and the Census's own American FactFinder (AFF) for quicker access to data.
Basic household income by dollar amount brackets is in table B19001 CR |
However, since income varies widely from place to place, income quintiles can be a better way of comparing between places. These tables can be helpful
- B19080: Household Income Quintile Upper Limits
- B19081: Mean Household Income of Quintiles
- B19082: Shares of Aggregate Household Income by Quintile
CR | SE | AFF
Another generally cross-comparable statistic is the Gini Index, which is reported in B19083 CR | SE | AFF
You may also want to look at this page from the Census Bureau which goes deep on all things about income.
*As with other American Community Survey data, US and individual state data can be found in annual "1-year" releases, as can data for places of population 65,000 or greater. (This includes about 25% of US counties)
Data for places of population 20,000 and higher have been offered in ACS "3-year" releases (estimates are generated from data collected over a three year period), but this release has been cancelled effective this year.
Data for all Census geographies regardless of population is included in the ACS "5-year" releases. The data collected for the ACS 2010-2014 release will come out in December.