easterntraveler:
Pipe shows are also another good place to find vintage blends.
You could also post a request in the Want Ads section of BriarBids.com, if you're looking for something specific. BriarBids has also just opened up a section for pipe tobacco trades and sales. Worth checking out.
Check out pipestud.com, who always has several pages of vintage tins. If you're looking for Dunhill blends: http://www.bruyere-pipes.com/vintage_pipe_tobacco.html (although you won't find any bargains here!). Chief Catoonah has a lot of vintage McClelland: http://www.chiefcatoonah.com/mccle-vin-tin.html.
I've bought a lot of discontinued tobacco on eBay and have never had any issues. But, yeah, there are guys selling tins with a year or two of age on them that they're calling "vintage" and "rare" or "discontinued." So just be sure you're looking at what you think you're looking at.
Bob