I highly recommend you use Korean red pepper flakes because it gives this salad the beautiful red color it's known for. Korean red pepper flakes are more fruity, earthy, slightly sweet, and can be less spicy depending on the brand you get. It can be found in most Asian grocery stores or in the Asian food aisle of your grocery store. You can substitute with other red pepper flakes but you won't get the vibrant, intense red color to the dressing. I would reduce the amount to half to start with depending on how spicy the substitute is.