A good site, at least to start with, is
http://english.songtranslator.net They have very few translations now as they've only just started, but they allow you to make a translation request and promise to have it done for you in 24 hours. From what I've seen they're very good, although if you check out the main page-
http://www.songtranslator.net- you'll notice that the site dedicated to translations of English songs into Spanish is a lot healthier