Soil data sources
SSURGO description here
It exists in shapefile and gridded form (SSURGO and gSSURGO)
STATSGO - description here
There are a variety of ways to get these data:
Web Soil Survey is a web-based download system that allows users to select a region of interest and download soil data layers. ROIs have to be pretty small.
Geospatial Data Gateway allows downloads of gSSURGO, SSURGO, STATSGO2, by state or more specific areas.
NASIS may be another option