Get something from the store. I have used the muriatic acid though. Didn't help on the overheating much, but did soften up the scale so it was easier to rod out the radiator.
If you're just doing it as a preventative, I would use one of the commercial radiator flushes. Most are citric acid/citrate based these days. They are safe for people and the radiator and are reasonably effective, though you may want to allow a little longer run time than the muriatic acid recomendation. You could take out the drain valve plug or unscrew the temperature gauge probe to gauge when you have removed all the scale.
Thanks. I don't have to do it to the block or head, because I've just finished rebuilding it and everything has either been hot tanked or washed with solvent under pressure, but there is still the radiator to think about. How do I clean that by itself so I don't run all that dirty junk through my nice clean engine?