Arbor Vacuum has two locations: one on Packard Street between Wells Street and Woodlawn Avenue in town, and one at Carpenter Road. In addition to vacuums (sales and service), they're also a good source of high quality blenders, food processors, knives, and assorted other home machinery.
Their website is http://arborvacuum.com.
1226 Packard Street, Ann Arbor - (734) 761-3677. Doesn't look like they're open Sundays.