Pellets you can find in the plastic bottles of Bars Leaks at your local WalMart. Not the absolutely cheapest version, the next step up in price. The bottle is clear in some areas, be sure to shake it up so all the little pellets roll out into the radiator.

For the head gasket water leaks I would suggest that blue Rolls-Royce stuff (forgot the name but it is better than silicone for this application.)

R.