Making them is definately the best and cheapest option. Then you can put in the weight that suits you best, from heavey to light.
Things I have known people to have in sock poi include... bouncy balls (my personal fave), juggling balls, tennis balls, balled up socks, rice, pasta, lentils (all sealed in bags or, yes, you've guessed it, more socks).
Also, many people find the Hop socks too stretchy for their liking, some like them, some don't, all down to personal choice.
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Live like there is no tomorrow,
dance like nobody is watching
and hula hoop like wiggling will save the world.
“What lies behind us and what lies before us are tiny matters compared to what lies within us.”
Ralph Waldo Emerson