Hello there EJ,
First, check out our glossary page on antonyms, which contains some examples: http://www.usingenglish.com/glossary/antonym.html
I had a quick search for you, and came up with a couple of online dictionaries that give antonyms:
And if you want to look for more dictionaries that I have missed out, here is a great place to find them:
I hope these help, and if you need any further help or advice, please feel free to ask!
- For Teachers