Where can I find it near Champlin?
It depends on your standards and what you mean by sourdough and bread. Lunds/Byerlys bakery would have something labeled as such, but it's a very poor example. Probably whatever you could find at Super Target likewise.
The closest possibility I can think of would be Chez Arnaud in Maple Grove or Plymouth. Most of their breads are not naturally leavened, but IIRC they do a few varieties. Their baking schedule and availability varies by day.
The only real (and good) artisan sourdoughs I have had here are Patisserie 46 and Rustica. Both in Mpls.
I'm not familiar with the food coops in your area, but many of the coops closer to the cities of Minneapolis/St. Paul carry Saint Agnes bakery's sourdough loaves. There is Grass Roots Coop in Anoka. Might be worth calling them.