Yes, frogs are sold everywhere, alone or standing over an skull.

If you are student you'll need one....

The story is that if you are able to find the frog over the skull in the front face of the university entrance, you'll have luck with your exams. Try it, it's not easy, the frog is really tiny and the façade is overdecorated (plateresque style).

This seems to work for my girlfriend who weared an small silver frog in a chain while she was studying.