They have it all over at cart stations across the park
They have popcorn stands setup in pretty much every section of Magic Kingdom. Checking with the cast members is your best option if you can't find any.
Yes there are a lot of options down there, near jumbo
Last time I was there it was around every corner pretty much. You should be able to find one pretty easily walking around Main Street. Just ask a cast member while you're there.
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