If nothing exciting was written in Greek or Latin, neither would have become very popular second languages in medieval Europe. A good way to kick start your language is to translate good material into your conlang. Public domain is a good place to start, but the content in public domain isn’t always the most interesting: VOA news articles, some government documents, really old novels.
Creative commons texts are another good source, but only if they are not ND (no-derivatives).
“In general, a translation from one language to another or a film version of a book are examples of derivative works.” CC Faq
Translations are derivatives. You can translate and republish ND content if you get the specific permission from the original author.