The house I'm renting has some 110v audio equipment that I'd like to use and the transformer that they have seems to be fried. I'm looking for a shop where I can buy a transformer at a reasonable price. If it's close to Subte Ministro Carranza that'd be a bonus, but I'm willing to travel.
I think many electric hardware stores have these items, walk a couple of streets in the center and I guess you find a couple. Otherwise the feria (market) might also have stands that sell that sort of stuff.
If all fails, here I have a link to MercadoLibre, the ebay of latin america, they have plenty of these items for sale in the capital: transformador 220v a 110v