If you want to purchase the blackboards directly, check either Librer?a Rodali or Librer?a Ripoll. Rodali is between Doce de Julio and JF Kennedy near the photography studio of Ted Jim?nez. Ripoll is on Doce de Julio about a block before the post office.
Or, if you were going to Santiago, check out a store called Educa. They sell desks, chairs and all sorts of school supplies - mostly to private schools. I have the telephone number at my office, so if you are interested and speak Spanish PM me and I will get it for you.