Years ago, we used bags of dry concrete, wrapped in heavy duty plastic bags and several strips of duct tape. They take up a bit more room, but they are cheap and easy if you are only wanting to add a couple of hundred pounds.
I have 4 steel blocks that are 8"x11"x2" which I had handles welded on the tops and rhino lined. They weigh approx 50lbs each and they cost me $ 140.00. Got them Rhino'd for free. Work great for moving around to balance or weight one side with surfing along with my side sacks.
I called around to a bunch of "mom and pop" tire stores around Indianapolis about 8 years ago and ended up getting 1200lbs by the end of the day in tire weights. I melted a few down into bricks but that takes too long, just go to lowes and get those small canvas tool bags they hold about 50lbs of them. I think I only ended up paying out 80$ by the end of collecting all the lead. Some of these guys had been saving it for years! Grab some gloves and fill up bags!