steam boiler additive

I've read on brownstoner and a few other sites that it's a good idea to add some cleaner to a steam boiler to get rid of the sludge and rust.

There's stuff made by Hercules/Scout and another called SOS boiler colloid.

Any idea where I can purchase them? Hasn't come up on home despot or lowe's websites.

http://www.gothamchemical.com/index.htm

I use these guys. They actually set up a schedule and service the boilers. I don't know if they simply sell the stuff.

cool. thanks