Sorry if this has been asked before, but I couldn't find it if it was. What, if any, are the standard rules for players drawing a new investigator, once all the investigators have already been used? Do you just start recycling used investigators, or are the played eliminated when their investigator is defeated?

It's in the reference guide, pag. 4, second bullet point under "Other Defeated Investigator Rules" (emphasys mine):

When an investigator is defeated, the player chooses a new investigator at the end of the Mythos Phase. A player may not choose an investigator that has been previously defeated during the same game. If the player cannot choose a new investigator because there are none available, he is eliminated.

What Julia said, though I can't really fathom a situation where it would happen and you haven't lost by then.

Agreed. Especially now that you have quite a wider pool of investigators. Maybe it's possible in very large group games, dunno; it never occurred to me in any of the games we played

I completely missed that. Yeah, now that I've picked up MoM and FL (at all of two days after I picked up the core game, because I had the money and it was just that good), I'm not as worried, since there are 20 investigators. My worry was more if you had a full 8 players and just the core game and 4 investigators died. At my FLGS we tend to ramp up difficulty in Mythos games, so that's not a far fetched possibility.

I play ironman version, where we go against each ancient one, and if an investigator defeated, we can't use them for the others as well.