Here's another source of money -- us.

Retirement Plan Changes As 401k Contribution Limits Increase

These funds are invested in stocks, bonds, or both, mostly in equities. This is an infusion of cash into the stock market, and it's being contributed willingly by those who pounce on this 'opportunity' to hand over more of their earnings to Wall Street.

It works if you retire and cash out when the market is up. It doesn't work out so well otherwise.