I would use a marble
I am assuming this is a tiny model, so a marble or any smooth sphere object would work great. I chose this because the earth is actually very smooth, it only seems like it isn't because we are so small compared to the Earth. look at a table, it probably looks smooth. If you use a low-power microscope you will realize the table surface is actually very uneven and rough, but we don't notice this because we are so large when compared to it. Same thing with the Earth's surface. So yeah, anything round and smooth to the touch would work.