It's online several places, including the New Advent website (LINK).
A source 283 years after the Apostles died??
Didn't I supply a link to you a few days back that explained how that was?
Most of the ECR's Jerome cited can be noted for their LACK of mention of PVM (perpetual virginity of Mary). Chew on that one for a while. Seriously.
Even Clement does not ever say that Mary was perpetually virgin. I just read what he wrote on the subject. If he had thought that, among what he wrote stressing virginity as being (part of?) a superior spiritual condition -- then why did Clement NOT make more of an example out of Mary? He touched upon her being virgin, and giving birth to Christ, then quickly moved on.
Here again it appears to me that you are following in path of Jerome error, in this. Helvidius was not the first, nor the only Christian to assert that Mary and Joseph had other children -- after the miraculous "virgin birth" of Jesus, of course.
As late as the earlier 4th to mid 4th century, St. Basil of Caesarea (330-379 AD), although himself believed Mary to not have given birth to children after the miraculous birth of Jesus, mentioned that the view that Mary and Joseph did have children together was widespread, and was not outside of orthodoxy. That last part-- "not outside orthodoxy" is almost a direct quote. It's the idea of it that is important.
As such, it serves as a way-point of sorts for the development of the PVM doctrine.
Let's all face the facts here, shall we? If PVM was in actuality a teaching coming down from the Apostles -- there would not be need for RC apologists to revert to relying upon one of Jerome's worst, ill-founded argumentatives, or else treating Protoevangelium of James as if it were anything other than the unreliable, fanciful pseudograph that it is.