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I'm in SE Michigan.
I think if you could find some right away, within the next couple of weeks, they would be fine. By the time they're 3 months old, they should be able to handle pretty much anything Mother Nature can throw at us here in the Midwest/Great Lakes, assuming they have proper food, water, and shelter. If they were hatched in August, they would be 3 months old in November before the worst of the cold hits.
I think the other important factor would be to have them used to the outside right away, so that as summer transitions into fall, they can gradually adjust to the cooler and cooler weather. Try to get them out of the warm brooder fairly soon, by the time they're like 4 weeks old, I'd say, at least partially -- they can still have a warm spot to move to when cold, all winter if you like, but they should be mostly spending at least their days in the ambient temperatures. I think if you kept them indoors and warm and then suddenly put them outside in colder weather, like some frosty October nights, THEN you would have problems. But if they are used to it gradually, it should be ok -- kind of the way you never just dump live fish from one temperature water right into another, but float the bag a while to allow temps to equalize.
Good luck, hope you can find some chicks on fairly short notice.
If not, what some of the other posters suggested was good -- try to find some older birds. Look on craigslist or hoobly.com.