Thangles is the brand name of a pattern for making half square triangles. they're papers with pre-printed sewing and cutting lines. they come in a few different sizes. i think you also have a choice between sheets and rolls. you put the two fabrics right-sides together, lay the thangles on top, sew on the sewing lines, cut on the cutting lines, tear away the paper, press and voila!!! half square triangles.
you get gazillions in each package. last time i looked the price was reasonable. i make my own because it's easy; it might be less expensive, but i've never done the math to confirm that. but i don't have to wait for shipments; and i can make them any size my little heart desires with just a few keystrokes and mouse clicks.