I have been using the Online Declination Calculator from NOAA's National Geophysical Data Center when I need to find the declination for an area.

http://www.ngdc.noaa.gov/geomagmodels/struts/calcDeclination

from what I can tell it accurately covers a local park that has a declination 1.5 deg greater then just a few miles away. The values compare favorably with several major land marks you can see at quite a distance thanks to the local terrain.