A decent online source is poolgeek.com and they sell the following (prices are for 25 pound size) GLB 3" Trichlor tabs at a decent price ($91.14) and these are 99% pure Trichlor. You'll save even more if you can find these tabs in a local pool store since shipping is extra ($9.99 for 25 pounds). Other brands at 99% include Omni ($102.25), Robarb ($86.57 that appears to be the cheapest), ProTeam ($117.08).