Sometimes you can find drain cleaners that are pure Sodium hydroxide (NaOH) at home improvement stores (check the label). May be cheaper to order it from a chemical company but if you need a small amount that would work.
Just be sure to use safety precautions. Its a very strong base.
When I went to find it at Lowes, they didin't have it with the other drain cleaners in the cleaning supply sectio. I had to go back to the plumbing section to find it, also check on line for availability, or just order it online from Lowes.