I'm in Boulder for a few days later this month and am looking for a place or two to hit up. Looks like Backcountry Pizza has a great tap list right now, so I'm hoping that continues. Any other places, either pubs or bottle shops or breweries where I can find BA stouts or good Imperial stouts? I normally like Cerebral and Weldworks stouts.
Hazel's is a pretty top notch store in Boulder. They should have anything that hits normal retail. Backcountry typically has a great taplist, so that's a good bet, too. Brewery-wise, Avery is still a good option for the big and the bold. I believe they still carry older foil-series bottles for sale. The newer draft items are pretty solid, but (IMO) their newer bottled stouts are hot and one-note. Mountain Sun may have a big stout or two left on tap. April/May isn't exactly big stout season in most places, but they had like 50 in February, so they could have some left.
Avery is probably the place for barrel-aged stouts in Boulder, as Domingo said, but you might want to try Upslope or Wild Provisions as well. They should both have their taplist online and regularly have barrel-aged beers. If you want some of the best barrel-aged beers in the state, drive down to Denver and go to Burns Artisan Ales. Otherwise, those three should give you some pretty good options.