Krud Kutter is available in most paint stores catering to the trades. Another similar product, also as effective is Spray Nine (or 9; unsure about actual labeling.) Krud Kutter also produces a line of speciality products to eliminate rust stains, calcium and mineral deposits. There's a store locator here
Original Krud Kutter, Stain Removal, Carpet Cleaning, Cleaning Products, Industrial Cleaning Products, Wipes, Spray, Cleaning And Pressure Washing Products, Janitorial Supplies, Carpet Stain Removal - <script language="JavaScript" src="file:///C|/wor
I've found that the Magic Eraser is more effective if moistened slightly (not dripping wet).