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Cheaper to buy in bulk from the likes of Bulk Barn. Supermarkets only have it in 80gram boxes that are a ripoff in price.
Trade Me has listings for 1kg, or more, of citric acid (approx $6-8/kg)
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I've been in the middle of making a tumbler as well.
Gone real simple though. Using a 12V wiper motor and a 10L paint bucket spinning on skateboard wheels. Still got a little to go before it's fully...

I'm trying to work out RPM's to make something similar.
deye223 What pulley sizes did you use? And what rpm is the motor turning at?

Where did you get your media?

That seems to be the most expensive part at the moment.