Where to buy cheap electronic bits in Mexico City ?

I needed a Europe-to-USA power adapter. I entered the first Radio Shack shop I spotted in a mall and found what I was looking for for... 89 pesos (> 5€, USD6.5), so I didn't buy it. Same mall, another shop called Steren, and there, bingo ! Same little adapter for 14 pesos (0.8€, USD1) ! I bought 2 of them.

European-American travel adapter(s).