NAUGHTY HB, .... naughty!!
Life gets difficult having to be inventive in this country - don't it??
Try HB's suggestion but
1) wrap the cardboard or hardboard in clingfilm (when the paint is dry)
2) have the sign plasticised (heat wrapped)
3) use plastic whiteboard instead of cardboard.
Failing that, go to the libreria (stationers/bookstore/cardshop) Carina mentioned in Duarte OR go to Ripol's store on Doce de Julio. Ripol's son works there & speaks quite reasonable English (found THAT out after chatting in Spanish for 18 months!!). ~ Grahame.