The magnetic declination can be calculated. There are programs you can get on the web to do it for you.
I use Geomag, located at
http://web.archive.org/web/20060710...oaa.gov/seg/geomag/data/geomag/geomag_v60.zip
THis url also includes a C program source.
This used to be available at one of the NOAA web pages, but they have switched to on line calculators, so I've included a archived link to the old file. The calculators are probably easier to use anyway. One of the calculators is at:
NOAA's Geophysical Data Center - Geomagnetic Data
The Geomag program usually only works for 5 years at a time, at which time you need a new data set. The one with the program above is good for 2005-2010, one more reason to just use the online calculator.
It's amazing how much these declinations can change over time. I think my magnetic deviation has changed a couple degrees since I moved here, I think it was above 17 deg back in the 90s, and now is less than 16 deg. Another reason not to use maps, because I'm guessing that a lot of those maps are not re-drawn on a regular basis.