Most governments will provide a list of agricultural approved chemicals, which will include the active ingredient, its concentration and the trade name of the product. In Australia, the Australian Pesticides and Veterinary Medicines Authority maintains a large database of all approved products. You can manually query the database. I don't see a programmatic interface, but I think it would be possible to write something to automate the queries and scrape the returned html (but it would be a bit of a pain).