I think there's a few aspects here --
- "authentic" as in, they really cook this dish in that region.
- "traditional" as in, they've cooked this dish in that region for quite some time.
Of course, traditions develop over time, and cooking techniques and ingredient availablity changes over time, so a "traditional" dish might only be a century or two old.
If you're concerned about recent corruptions due to more recent changes in globalization (eg, overnight shipping of produce, external influences), I'd go with old cookbooks. Of course, I also volunteer at a library managing the sale of donated books, so I have an easy and cheap source (just got 3 more yesterday). But check at yard/garage sales, car boot sales, flea markets, estate sales, library book sales, used book stores, etc.
For US regional cooking, I like church and community cookbooks; for foreign cooking, I think I have the majority of the Time Life "Foods of the World" series that was published in the late 1960s.
And if you want to get really picky about "authenticity", you also have to consider that many of the great recipes in a region came from peasant dishes. Is it being authentic to the dish to get ingredients flown in from the other side of the world to make it, or would it be better to find a reasonbly priced fresh local alternative?