You can use any square ruler that is the size. If you don't want to invest in the ruler then make a paper pattern the size and use it as a pattern. Fix it to the fabric and then use the rotary cutter to cut out the shape. I have several square rulers and they all have lines in which you could line up the corner for the triangle. I put some kind of tape on the bottom edge to be able to alighn up correctly. Not a problem and you don't need an expensive ruler to do it.
The large ruler I have retails for just under $50 which is a lot of $$ for occasional use.