It's a while since I've bought one of those triangular ones, but Hacketts in Baggot Street or Sullivan Graphic Supplies in Grantham Place (off Camden Street) should stock them. I'm pretty sure Easons still do a bit of that stuff or any place that stocks school supplies. As a matter of interest why do you want the triangular version- these are mostly favoured by surveyors and those that work with maps or large sheets, most architects prefer the more common oval section flat scale rule, it's easier to carry and doesn't get damaged as easily. I use the oval section variety myself.