Muiratic Acid is available at most hardware stores and even at some Walmart stores. Normally it is used on cement/driveway/brick as a cleaning agent and can be bought in 1gal sizes for $7-$12.
What you are buying is a dilluted form of Hydrocloric Acid. It will eat away rust quickly, heavy paint over a few hours, and any metal given enough time. DO NOT use it on Aluminum as it will dissolve it away in short order.
MSDS fact:
http://complyplus.grainger.com/grainger/msds.asp?sheetid=3760004HAZARDOUS DECOMPOSITION PRODUCTS:
FLAMMABLE HYDROGEN GAS CAN BE PRODUCED BY REACTION WITH MOST METALS AND MAY FORM EXPLOSIVE MIXTURES WITH AIR. CHLORINE GAS WILL BE RELEASED BY MIXING WITH STRONG OXIDIZERS.
Use generous amount of water to rinse.
Be aware that if you do use Muiratic acid for cleaning you need to apply a protective coating to the bare metal because it will rust back.