In my mind, as national breweries go, New Belgium definitely has a leading sour beer program. There are some much smaller breweries making a ton of great sour beers, but NB sours are easier to find. Overall though I'll throw out my other favorites Jester King, Crooked Stave, The Bruery and Rodenbach to name a few. Boulevard Love Child series is another sour I never miss.
But, back to NB, here's a couple of good videos from them on sours that I thought I'd share.
"Brewmaster Peter Bouckaert reminisces about the origins of New Belgium's sour beer program, which started in the late 1990s."
"New Belgium brewmaster Peter Bouckaert talks about the history of sour beer."