First of all, keep in mind that, as with the name Jesus, many people have had the name Valentine. The oldest source I have found so far for a St Valentine is the Martyrologium Hieronymianum, a list of martyrs. Google Books has a fully-readable online version of it right here. Its name suggests that St Jerome compiled the list, but that is no longer generally believed. The Martyrologium Hieronymianum was probably originally written in the 5th or 6th century, but in all likelyhood it was re-written over the next few centuries to conform to the popular legends of saints. So. As far as I know, this 5th, or 6th, or 9th century list of saints is the oldest source of information about a St Valentine. And all the Martyrologium Hieronymianum says about him is that he was from Rome and that his feast day is February 14th. Nothing at all about Claudius II or marriages. The earliest account I've found of interaction between Valentine and Claudius II is the 13th-century Legenda Aurea of Jacobus de Voragine. You can read that on Google Books also. I'm not claiming to be the best researcher in the world. (Although I'm certainly not the worst either.) If someone has more info on the early sources of Valentine than I do, that'd be great.