Check out this link:
I bought 500 pounds of weight from them. It isn't lead, but steel shot. The bags are great, they are super sturdy and conform easily due to the steel shot. It will take up more room than the equivalent weight in lead, but is still compact. I am really happy with these, so I'd definitely recommend them. Plus, shipping was a breeze. In total it was $500 for 500 pounds, shipped.