JP is right, many of the small neighborhood hardware stores carry it. Another good resource for electronic equipment, especially computer components, is the Jardin Florida. Its a gallaria-style shopping center on Florida Street between Lavalle and Corrientes. There are 40-50 small shops with a wide variety of stuff. The prices are better than anywhere else I've gone in Buenos Aires, but they are still 50-100% higher than the US due to the import tarriffs on electronics.