'Whence' is a good alternative to 'where' in this sentence. Where is still used correctly.

Your question is worthy of a grammar overhaul. However I believe your question may be: if one can not find a word in a dictionary, where can it otherwise be found? ANSWER: All currently accepted words in English are in the Merriam Webster Dictionary, however various words from particular vernaculars like Ebonics are likely not listed, yet it is important to note that languages evolve and if a character speaks using unlisted words then perhaps simply using deductive reasoning to determine the definition.

"Proper" is not necessarily a rule of grammar and more of a condition of personal preference. " So" is a conjunction....so it is always correct to utilize it.. especially if the writer is writing dialogue... Then it's just a manner of a character's speech.. regardless if it is context of a particular vernacular...and thus there are no ' proper ' syntax rules applied.