You can get a small container of them from hobby suppliers for an outrageous price, or you can mail order a 1 gallon sized package of them from Wicks Aircraft Supply, or Aircraft Spruce. It will cost about $6.00 for a 1 gallon bag, vs about $6.00 for an 8oz bottle from the likes of Tower.

They are very small spheres of phenolic glass. I am not familiar with how they are made, but I doubt many of us have the equipment it takes.

Safety caution: use a dust mask when handling and sanding this stuff.

Vince