Translations of arias, songs, and sacred vocal music may be found in several resources. Links to the Leyerle (aka Nico Castel) publications and some of the most popular and reliable websites are below.
But one can also find translations in
Western Libraries also provides access to bilingual dictionaries, which offer translations between English and other languages both in print and online. E.g: English-Italian/Italian-English; English-German/German-English, etc.
If you use a translation from any resource - either for a paper or a performance - remember to cite it!