Source for US parcel boundary data?

Data for US federal lands can be found through the GeoCommunicator site. However, it does not cover the entire country.

For private parcels data are much less accessible. For these data you generally have to go to the GIS department for the county, or if there isn't one you may even have to digitize paper maps/plats.

Some private companies, such as maponics, which provides data to Zillow.com, have amassed a good database of private parcel data, but these data are not free.

Report All website has a large data archive of parcel data per state/county (some states and counties are not included). You can purchase data per county via shapefile, kml, or excel.