HCl can usually be found in building supply stores under the name muriatic acid. It is used to remove grout stains from brick work and to clean concrete and stone. Expect to pay around $8 per gallon, but sometimes you can find 1 liter bottles for around $3.
Chlorine test strips and chlorine dioxide test strips react to both chlorine and chlorine dioxide. Glycerin can be used to get rid of the chlorine so when looking at chlorine dioxide you measure the concentration of the sample, add some glycerin to eliminate the chlorine, then measure the concentration of the sample again. The difference between the measurements will give you an indication of the amount of chlorine present.