I've used sacks of coarse aquarium sand for Dove and Pheasant. I shopped around until I found an acceptable price of $10 for 50 pounds. Most birds seem attracted to light (white) spots left in fairly open area.

Any Poultry supply is likely to have Turkey grit, Generally speaking, the larger the sack the cheaper the product. But like I said a light color seems to draw them quicker/better.