Most art supply stores sell it.
I know the one in my area, on Bustamante y Santa Fe, has a whole section.
But do you want PAINT, which is for drawing images on fabric, or DYE, which is for coloring the whole garment.
In english, these are two very different things.
Dye is available at most any Merceria, there is usually one about every two blocks, selling thread, undershirts, buttons, pins, and pajamas, usually. Also, often, aprons for Mucamas. Why that mix, I dont know, but virtually every one I have ever been in has that selection.
Also, of course, Once. A block or two towards Santa Fe, parallel to Corrientes, and towards the microcento from Pyurredon, there are several big notions stores with huge selections of both paint and dye.
This place has 4 branches in Once, they carry the major brands- on this page, you can see the difference between paint/pinturas and dye/anilinas.
http://www.casajorge...-y-quitamanchas