I scraped almost every pokemon cry from pokemoncries.com save for ultra beasts, and the gen 8 dlc legendaries. There's a readme file included in this zip that tells you which dex numbers are missing. I also couldn't find a source for alternate form cries unfortunately.
There's both old cries (gen 6 updated every previous generation's pokemon cry, so the old-cries folder has those) and (almost) every updated pokemon cry in the new-cries folder. The wav files are already properly named.
pokemon-cries.zip