Cisco Lady of the Woods - Green label, white writing, bomber sized bottle with a caged cork. Sour as ever, pale, highly carbed, rockin' head, with tropical fruit notes and light oak. I'm surprised no one has mentioned them so far being a Mass. Brewery and all.
Anything from Cantillon is amazing. Fantome makes some good, funky brews but they're not extremely sour per se. You can get Oude Tart from The Bruery and Hanssens Oude Kriek on the East Coast, but they are just "good" IMO. The Jolly Pumpkin sours take awhile to become good and fully carb up. I've had more than a few flat ones. Keep your eye out for Captain Lawrence sours, and German Berliner Weisses if you can find them.
You'll find that most of the excellent sours with a decent price are on the West Coast. Russian River, Lost Abbey, and Cascade Brewing make some of my favorite sours. You'll have to trade for those. Jackie O's out of Ohio also makes some excellent sours.