There are several online resources for finding synonyms and antonyms. One of the best-known is Merriam-Webster, which hosts a website that has both a searchable dictionary and thesaurus along with other useful tools.
Another famous synonym and antonym finder is Roget's Thesaurus. It, too, has an online version that works in a similar way to the Merriam-Webster finder. Both tools allow the user to enter a word into the search function and then view the resulting list of synonyms and antonyms. Both resources provide a list of related words, and the Roget version permits the user to apply criteria such as relevance, complexity and length of resulting words.