I get the audio files directly from League itself. I use a program called Ravioli Explorer to unpackage the WWISE .bnk archives that contain the quotes for every champion and the extraction and file conversion process is also handled in the same step. Unfortunately it's not quite advanced enough to also be able to decipher the development name of each quote, but it's the only tool that I and most people have access to at the moment, so I have to make do with it.
Ah, I made a mistake. It's the WPK files that contain all the quotes for champions. You can tell that's the case since the WPK files are massive in comparison. I don't know what the BNK files are for but they might be for triggering events for those quotes - trying to open them will just get nothing.