Pindo,
Have never purchased from them, but they have a long list of products for the organic gardner in 1,5,10,20 pound quanities, including Greensand.

Here is a link that might be useful: organic products

If you're talking about buying a truckload, you could call Garden-Ville in San Antonio. When they can't help you with huge orders, they'll put you in contact with their supplier.

I know that Agway stores carry it in large quantities!

Ron
The Garden Guy
http://www.TheGardenGuy.org