One good source for finding antonyms is Thesaurus.com. Search any word on the site's home page search box to find a list of synonyms and antonyms for that word. Alternatively, use OxfordDictionaries.com. Click the Synonym tab on the home page, enter any word in the search box, and find a list of synonyms sorted by usage with corresponding antonyms listed below. An antonym is a word opposite in meaning to another word.
To filter the antonym search results on Thesaurus.com, use the Relevance, Complexity or Length sliders to sort results based on those criteria. Click the Common or Informal radio buttons to see which antonyms are commonly used in formal written and spoken language and which are part of more casual conversation. Click on any word to see the definition.
To find antonyms on OxfordDictionaries.com, look under the synonym results, which unlike Thesaurus.com are sorted by usage and context, accompanied by an example phrase. Click on any word in blue to see the definition.
Another option to find antonyms is WordHippo.com, which allows for the direct searching of antonyms. Use the search box labeled What's the Opposite Of on the home page. Click on any of the search results words to see antonyms for that word.