It depends what size you want. I have some dress making pins with a round plastic ball on the end. it pulls off easily & comes in handy for just what you're doing. They are about 3/16 ths of an inch in diameter.
OR, ball bearings? drill a small depression into the model surface & superglue into place. OR, a small ball of filler, rolled between the palms of your hands.
Think laterally, when scratch building, anything & everything becomes a potential part of a model!
Pete