I need a flash big enough for my Canon Rebel XT to look over my wide-angle lens?

I just got a Tamron wide angle zoom lens 17-55mm f/2.8 and whenever I take pictures with the flash on there is a dark area at the bottom because my flash isn't big enough to look over the lens. I need some recommendations for some good cheap flashes, either Canon or a brand that will fit on my camera. Thank you.