How do some t-shirt companies manage to use licensed characters?

Let's say you go to a mom and pop t-shirt store and find a Garfield t-shirt, but it wasn't made by the people who make Garfield. How do the t-shirt manufacturers get the license? There are countless other characters on t-shirts everywhere: Snoopy, Disney characters (on non-Disney brand t-shirts), Scooby Doo. There has to be a method for getting an authorization to create merchandise with these characters. If you can point me to the right direction please do...