Call around, you might get lucky. There’s also a product called “Liquid Fire”, which is marketed as a last-ditch drain opener for use when all else fails, but is actually concentrated sulfuric acid. As far as how much to add, I’m no chemist. You might also take it to a battery shop and see if they can help you, as they would have the chemicals and handling equipment, and know how much to add if you’re lucky.
If you do find some acid and choose to do this yourself, make sure you wear wrap around goggles and acid resistant gloves that cover your forearms. It may also be helpful to make a kind of smock from a garbage bag or similar to protect yourself.