There are thousands of resources online for great riddles, but since most riddles are a play on words, there are not automatic riddle makers online. Riddles.com allows you to find riddles pertaining to a certain subject matter, however.
Riddles are typically split between two types: enigmas and conundrums. Enigmas use metaphors and allegories to require careful listening and ingenious thinking from the listener. The sphinx's riddle from "Oedipus Rex" is a great example of an enigma. Conundrums rely on puns in the question or the answer. "What runs around a city but never moves?" is a great example of this. Because of their language-based difficulty, riddles sometimes are difficult to translate.