How can I aquire (not avoid) sour blueberries?
Blueberries don't continue to ripen after picking so finding those that are blue and tart can be a challenge. If you see berries that are slightly purple or red, they may have been picked early and be a bit tart. 

If you're in an area where blueberries are raised, you might visit early in the season, which is late July to mid August. Picking your own might do it, or finding a wild blueberry patch. And if you're really into them and have the ability, consider planting your own bush; early Spring is the time to do it.