Sounds like you bought a car that someone had removed the clipper kit and installed euro bumpers, not stock US metal bumpers. I say this because I've tried installing euro bumpers with the US hardware and had to go out and buy the 8mm bolts for the euro brackets. My best suggestion is to take one of the brackets off the bumper beam (if you can, not real familiar with clipper kits) and take it to the hardware store with and try bolts until you find what fits.
BTW, post pics of the bumps you took off, or of the car before you took them off. If they are metal euro bumpers, please DO NOT THROW THEM OUT!! They are worth good $$!! Worth more than what you should have paid for a used clipper kit.