Which is the best website to buy funky t-shirts with interesting themes such as cartoons, football, movies, etc?

Google it !

amazon and ebay and etsy

Be careful if you are a seller. All the themes you mention are heavily watched for copyright infringement by their copyright holders. Sometimes you can get away with artwork that is different, and you are just using the names, but other times you can't. Disney is one company that will sue anyone using one of their names.

I'm sure there'll be a useful link posted very soon ;)