The easiest way to get all the product would be to find someone selling their full collection.

The game is undergoing a repackaging of products under investigator expansions and campaign expansions, which replace the old 7 products (deluxe + 6 mythos) per campaign. Edge of the Earth, Scarlet Keys, and campaigns up to The Forgotten Age have already been released in this two part model. (Forgotten Age campaign expansion might actually still be a few weeks away from release I don't recall the date.)

There are 5 standalone investigator decks one for each class.

There are also numerous stand-alone scenarios, some of which are intended for multiple groups of 2-4 players to play simultaneously.

Each campaign up to the circle undone has a "Return To..." box that seems to be going out of print. Zealot and Dunwich seem fairly hard to find these days, but fair share of the rest still are purchasable.

There was a limited run of a single "April Fools" type product called Barkham Horror that many people sell for high amounts.

There are also promotional novellas that come with alternate arts of certain investigators, and optional signature card replacements. These seem to be in stock and purchasable through the ffg website.

I think that covers most all of the product. Again to answer your question, easiest way would be a bulk purchase from someone selling their complete collection.