Raiding the weed farm at the Chiliad lumber camp... randomly gives you between 10K-100K. What else... maybe also the random event where you stop a bike thief and he gives you something like 100K of stock. There's another random event for money but it just makes a stock go up, so it's just a multiplicative reward... doesn't really matter if you do it before or after Lester's missions. Maybe cash suitcases? Those were 10K or 25K each IIRC. I mean, you get to a certain point where you really only need a solid 30-40M return on the Big One and full Lester rewards, and that by itself is enough to get you close to 2B or basically more money than you would ever need.
And, really, stocks can still go up and down afterwards, so technically you can still make money via that. I made a lot of my additional cash off of the BAWSAQ market.