I found something closer to what I was looking for (US State and County boundaries) on github: https://github.com/johan/world.geo.json/tree/master/countries/USA

The features are very generalized/simplified, but they'll work for a national scale map. Country boundaries for the whole world are available in this repo too.

You could use GeoServer to handle the conversion of PostGIS results to GeoJSON for you. A simple WFS request like this would return the geometry (and optionally the attributes associated with it).

http://astun-desktop:8080/geoserver/wfs?request=GetFeature&typeName=topp:states&propertyName=STATE_NAME,LAND_KM,the_geom&CQL_FILTER=contains(the_geom, POINT(-76.5617440647409 39.3951950048467)&version=1.0.0&outputformat=json