Probably most ferreterias would have them, as mentioned.
Here is a place that I am almost positive would have them:
La Casa del Transformador - Parana 220 / 4371-1023
They have everything electrical you can imagine. When I brought my brand-new Dell laptop from the States on one of my trips back and discovered that the idiots had given me the wrong cable for the portion from the wall to the transformer, I went there and they found a cable that worked with the three-prong "Mickey Mouse ears" connection my transformer used. Had the added benefit of having Argentine plugs.
But they had a ton of accessories and I'd be very surprised if they didn't have the type of power strip you're looking for.
And someone does have them here in BA - I have one myself, but I don't remember where I bought it about three years ago.