What is the triangle stock being used for? If it's just for strengthing a corner joint you could use any piece of triangle stock. If it's gotta be that unusual shape you'll either have to shape one by hand or change the part it mates to.

A balsa plane would make quick work of taking a piece of triangle stock down to the profile you need. Or as Planebuilder said, cut it from aileron or trailing edge stock.

Dave