Note to others - it will be more useful for the purposes of this discussion to list radiator shops that are still in business, rather than ones that aren't around anymore.
It used to be you could just look in the Yellow Pages to find them but that doesn't exist for most locations anymore. You may have luck by doing a web search on 'radiator repair near me' but you'll have to sort through the results.
A couple in the Minneapolis-St. Paul area:
Borgen Radiator Company
312 Johnson St NE, Minneapolis, MN 55413
(612) 379-2605
Tom's Radiator Repair
316 W Main St Suite D, Anoka, MN 55303
(763) 427-4294
www.tomsradiator.com
I'm not sure if this place repairs old radiators or just sells new ones
1-800-Radiator of Minneapolis
810 SE 9th St, Minneapolis, MN 55414
(612) 331-1221
www.radiator.com