Household object which are shaped like a triangular based pyramid are surprisingly uncommon! Although, Cheese graters, paperweights, candles and packaging are the most common items to be shaped like a triangular base pyramids.
If you have a children's toy box, then this may be the best place to look. Children's toys, like building blocks, often make use of the triangular based pyramid, or tetrahedron as they are commonly called.
If you can't find a triangular shaped pyramid you can always make your own. Here is an informative video which shows how you can make one out of cardboard, but you could also make one out of paper.
You can improvise and use whatever you can find to make your own tetrahedron. K'Nex or something similar would be a perfect building material.
