Where can I buy a good (effective) sweatband?
try checking @Baraka Sports or @Olympiad Sports and More
Where can I buy advent calendars but not chocolate?
Amazon.com is the place to buy these things from
Where can I find any places that serves hookah, on the DL?
Tough one, small under the radar cafes might serve shisha but I doubt you'll find any as it's a huge risk for the place to do so.
Where can I buy an umbrella with cool illustrations?
you can find illustrated umbrellas in places like @Accessorize or @Claire's