You can buy litmus paper at the Aromantic store, Supa Price, Johnsons Test Papers and Energise For Life. These stores offer litmus paper at a very affordable price. Litmus paper is mainly used to measure the PH of different substances.
Believe it or not, check with your local pharmacy. Many sell litmus strips for people who need to test urine or blood pH at home. There are also many school and scientific suppliers online who sell litmus paper.