Searching for "list Spanish antonyms" or "list Spanish synonyms" on Quizlet.com provides lists of user-created antonyms and synonyms in Spanish that are primarily from academic sources. Wordreference.com provides synonyms and antonyms as a part of its search function.
The lists on Quizlet.com are primarily for academic or studying purposes and come from sources such as the Spanish SAT, AP exam, and text book appendices. Wordreference.com does not provide a list of common synonyms and antonyms, but rather a search function into which you type Spanish words. The specific English translations for the synonyms and antonyms can then be found using the Spanish-English translation function on Wordreference.com.