We're in Des Moines, and it's cold here too. I put sweaters or a Tshirt and a jacket on mine when it's really cold out. My two don't wear booties-they HATE having stuff on their feet!
But, I grew up in the Minneapolis area, and had a Chihuahua, and she wore boots-they were the "Paw Tectures" brand from Petsmart. They are waterproof, and fleece lined, and stay on with Velcro-very functional and comfortable, but not so stylish. There are much more fashionable booties now, but these did a great job at the time, before they came out with all the super cute ones!
You might also ask Aprilove and Alaskayorkie about winter wear...Aprillove lives in MN, and I bet you can guess where Alaskayorkie is from
Another idea for you-try a 4-legged outfit to keep them warm. I have this jogging suit for Emma, and it keeps all 4 legs and her body warm but leaves room to potty too! (I got it from PetEdge last year, but they still have some available on Ebay!)