Another possible source would be Broadcasting Yearbook, which in more recent years has been known as Broadcasting & Cable Yearbook. I haven't seen a copy for a few years, so I'm not sure if it still goes by that name. They usually listed the sign on dates for stations when they were available, but I have found that the accuracy was hit and miss on some of them. Still, it's worth a try.
Yet another possible publication is the SRDS books. Again, I haven't seen any SRDS books for quite a while, so I don't even know if they are still being issued, or if they are still using that name. Check the reference section of major libraries, or the business and technology sections for them. SRDS was pretty good about listing sign on date information for most stations.