Sales of the Sprint iPhone are stronger at big-box retailers than at the Apple Store, according to new research. Citing a study by Consumer Intelligence Research Partners (CIRP), AllThingsD reports that Sprint iPhones made up just nine percent of iPhone sales through Apple, while it accounted for 19 percent of sales at Best Buy and 18 percent of sales at warehouse clubs and other mass market retailers. The difference was accounted for by stronger sales of AT&T and Verizon models at Apple, and comparatively weaker sales for those two carriers at Best Buy and other retailers. Overall, Sprint accounted for 12 percent of iPhone sales, compared to 62 percent for AT&T and 26 percent for Verizon. CIRP’s numbers are based on surveys of iPhone buyers over two three-month periods, ending in March and May 2012.