Will need to test this before using. I used to use Mars - miniature bars called "Fun Size" (19.7 g) and "Snack Time" (36.5 g) (both sold in multiple packs); a larger multi-pack size of 54 g; the regular sized single 58 g bar but that is expensive.
Make a little under half for the mass of the smallest to give the atomic number e.g. mass 28g, atomic number 13. You may need multiples of each and average to get a significant mass.
Use any of three sizes where possible:
jelly beans, but will need to test sensitivity of scales
choc chip cookies
Or two if you can't find threes
lolly snakes, pythons
Oreo minis, regular
Arnott's Tiny Teddy and Teddy Bear biscuits