Hi Ranunculas!
Isn't it terrible to live without quilt shops around?
There isn't much cotton fabric in Argentina, but if you want to buy stuff in general (fabrics, all sorts, thread, beads, etc) at good prices, go to ONCE. It's a neighbourhood in Capital, they sell mostly wholesale but you can buy small amounts too. Just go to Lavalle and Pueyrredón, and from there start walking along Lavalle heading downtown. There are infinite shops. Every now and then walk up and down some side street or just ask if there is anything special you are looking for
Good luck! Let me know what you find!