As is the case for most non-orthodox authors in the early church, none of Marcion's major writings survive, as the Encyclopedia of Ancient Christianity writes:
Since his writings have been lost over time, the information that we have on the life and works of Marcion has reached us through indirect sources, from the
numerous writings of his opponents.
Thus our primary sources are Justin Martyr, Irenaeus, and Tertullian, particularly Tertullian's Against Marcion, which frequently cites Marcion's Antithesis.
However, we do have several brief texts of "Marcionite" origin, which were likely written by either Marcion or one of his followers. They are the "Marcionite Prologues," short prologues to the Pauline epistles, found in some Latin manuscripts (including a sixth-century codex). They were first argued to be Marcionite by De Bruyne in 1907, and Harnack adopts his position, based on how they describe Paul's efforts in each letter. For example, the prologue to Romans reads:
Romani sunt in partibus Italiæ. hi præventi sunt a falsis apostolis et sub nomine domini nostri Jesu Christi in legem et prophetas erant inducti. hos revocat apostolus ad veram evangelicam fidem scribens a Corintho.
The Romans are in the regions of Italy. They had been reached by false apostles and under the name of our Lord Jesus Christ they were led away into the law and the prophets. The apostle calls them back to the true evangelical faith, writing to them from Corinth. (translation source; emphasis added)
It's by no means certain that Marcion actually wrote these prologues, but Harnack and others believe them to be very old and likely written by followers of Marcion – not his opponents.