where do they sell power supplies like that? Is it dell? did it come with that board also?

Yes it is a Dell server PSU 750w here is who makes and sales the boards. There was a group buy going on for them I just bought mine direct. You can get the board or the whole setup.

http://www.gekkoscience.com/products/D750_supply_breakout_board.html

Any trouble with running 2 s3's off of that 750?

None so far. All wires cool to the touch just as psu itself.