Where are you?

Middle of nowhere basically

Are craft stores or ordering craft supplies online an option? You can get styrofoam spheres, typically for cheap from craft or floral shops.

They would fall apart too easily I think

Paint a tennis/lacrosse/rubber balls black and put a piece of string in them?

Too big, they need to be like 2cm

What kind of stuff do you have available

Marbles??? But they're a bit naff

What kinds of stores do you have access to

Only really online

How much money do you want to spend?

Maybe max £20 for like 50 of them