I don't know where you are located, but in Barrio Chino, you have 2 places on Libertador. One is on the very corner of Montañeses y Libertador, they sell work clothes, rain boots, flip flops, etc. Old store. You might find the traditional ones with suela de yute (which is the rope mentioned above there). The other store has rubber sole alpargatas, and it is on Libertador between Montañeses y Arribeños, on the left hand side of the street if you are facing Barrancas.
And then, for fancy alpargatas, just look for Paez or ChimiChurri.