Yes there is. You need to search for carbon fibre based belt buckles. I can't remember which brand has them. But there are still metal in them. But i'm sure you could find them online. Or make use of plastic ones from girls range of belts. This one you need to be creative.

There are metal detector safe buckles. In fact there is such places that sells metal detector safe products. I just forgotten where. Could not find it in the sites again. Maybe i saw it in E-bay.