There are definitely dictionaries of slang, but you need to look at the specific region. Some Australian slang is impossible to understand in other English-speaking countries, and vice-versa.
I'd suggest the Lonely Planet phrasebooks of Australian, British or American English. Each of these looks at each country's own slang separately.
An easy-access website is www.urbandictionary.com, however be aware that the slang you find there may not be widely-known nor used often.
Best advice for slang is to understand it *after* you hear it used the first time. Looking for new definitions without knowing when and where to use them may take you in the wrong direction.