In my opinion you will be fairly limited to a handful of expressions that you understand and even less that you can adequately use. It would be better not to try to go local at this stage but concentrate on and put your energy into improving your 'normal' Thai and leave the slang and street language to the native speakers for the time being. However if you insist there might be books that others know about (I do not) and otherwise facebook, Line and Thai chat sites are an excellent source of slang expressions typically used by the younger generation.