I have only bought it by the bag. I also would like to know where to buy it in bulk if it is available anywhere.

There's an outcropping of Glauconite (Greensand) along Texas Hwy 1431 just north of Granite Shoals, west of Marble Falls. It's fairly crumbly and mixed with thin bands of decomposed granite. I scoop up a couple buckets full when I'm in the area.