I agree with Michelle's advise, but I'll give you another resource. While FFIEC's site is pretty good, the maps can be sometimes be rather, well, lacking. The detail sometimes stinks.
Here's a site from American Fact Finder, through the US Census Bureau. It is pretty quick and accurate. Just keep clicking into the area you need to zoom into. Odds are, if your property is "in nowhere land", there won't be a census tract boundary running flat down the middle of it, so as long as you get pretty close, the census tract will be easy to determine. You may have to contact the customer to find out some info on the property location (i.e. just SW of Littleville, between Dirtdigger Lane and Deadman's Ave.)
http://factfinder.census.gov/servlet/AdvancedGeoSearchMapFramesetServlet?_lang=en&_command=getPlacenamesAlso, on FFIEC's site, there is a "FFIEC census reports" link at the bottom of this page.
http://www.ffiec.gov/geocode/default.htmIf you click in it, you can at least find the state and county codes to go with your census tract code from American Fact Finder.
Granted, with this method you have to do a bit of on-line searching, but in my book, at least you still have your cushy chair, background music, etc., and no dusty plat books!
Good luck!