If you already have a perfectly serviceable external flash there's no reason to run out and pay a fortune for a commercial ring flash. Follow this tutorial to turn a food container into an effective ring flash.
Photography blog PetaPixel shares a simple tutorial for turning a food container (their container was a cast off from the counter at their local deli). You'll need a sturdy container, a razor knife, tinfoil, and a small metal bracket and bolt from the hardware store to mount your flash to the ring. The rest of it is as simple as cutting a few holes in the container, lining it with foil to reflect the light out, and hooking up the flash to your camera. Check out the full guide at the link below for step-by-step instructions and additional photos.
DIY Ring Flash with an Old Strobe and a Plastic Container [PetaPixel via UnPlggd]