I used Pop bags or something similar that they sold near me. They were 50 pound bags with yellow handles that I paid 40 bucks for and they worked great.
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!