We're making a vibration polisher for polishing laser-cut aluminum and steel parts, and we're looking for the abrasive medium to fill it with. We need about 30 liters of it - and the one we found (for jewelry use) was prohibitively expensive in the required amounts. We tried with several gravel kinds and the results were underwhelming.
What material, that would be easy to obtain and inexpensive is a common choice for a vibration polisher?