luckily my wife has discussed this several times with me. You will find the 20% solution at some of your finer organic farm supplies. Which she used to be manager of one of the largest suppliers for 6 years. Peaceful Valley Farm Supply, Grass Valley, California. while i don't like the owners that much, they do supply a good product, sigh.
She was also the organic farm advisor for the company, and people would ask for the weed killing strength vinegar.That's a problem. Apparently they can't sell it to you for weed killing purposes, but they CAN sell it to you for balancing the ph of your soil. So go balance the ph in the soil around those weeds will ya?