I've used
this site before for NASCAR Nextel Cup and Busch Series
and found them to be very accurate.
Are you heading to a certain race?
I would recommend, once you are at the track, look for the Racing Electronics trailer (others may have this too), get a Frequency Sheet for $5.00 and then compare them to the ones you have. I found that the teams will sometimes change the primary with the secondary, as well as a few minor changes. I think it's worth it to get this sheet.