Many in Texas, including myself, have found it extremely difficult to find higher concentrations of bleach. Apparently, there are some specialty stores that cater to the cleaning industry that may have it and if you live near one, it's probably worth a try. The highest concentration I have found is 10% at Home Depot, but take care to see that it is not exposed to direct sunlight and that it has not been sitting on the shelf for a long time as bleach will degrade under these conditions.