there are several stores in Once that have these in all sizes and shapes. Not sure exactly where- but really, you just have to walk around a bit, and you will see entire stores full.
There is a huge place on Cordoba and Scalabrini intersection ( I think it is called Margaritas ) They sell hair and makeup products for professionals, you will find it there. Or you can also try to go to Colombraro ( they sell everything plastic. Google them as they have many stores.