This may or may not be comprehensive for the Midwest, but it's all the ones that I'm aware of.
Minnesota:
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Minnesota Reptile Show - The first ever MN reptile show in like 2 decades or something took place earlier this year, and a repeat earlier this month (when I was out of town on vacation)
http://mnreptileshow.vpweb.com/
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Iowa
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Iowa Reptile Show and Sale - I went to this one earlier this spring, was small but good
http://www.iowareptileshows.com/
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Wisconsin
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Wisconsin Reptile Expo (W.I.R.E.) - another small but good one, I'm headed there next month
http://www.wireptileshows.com/
SEWERFest Reptile Show - Missed this one this year unfortunatly
http://www.sewerfest.com/
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Illinois
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NARBC @ Tinley Park (near Chicago) - Biggest one in the midwest, and WELL worth getting a hotel room and staying a night or two. The auction is great to attend.
http://www.narbc.com/
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