The ancient sources for Marcion were all compiled by Adolf Harnack in his Marcion: Das Evangelium vom fremden Gott, Texte und Untersuchungen 45, 2nd ed. Leipzig, 1924. Since Harnack died in 1940, all his work is out of copyright in Germany and the EU, and presumably in the US also, therefore.
Unfortunately this work is not online, as I found in a previous post. An English translation — Marcion: the gospel of the alien god — exists, but only covered Harnack’s introduction and not the essential sources.
I obtained a library copy of the book at the weekend, and started to scan it. Unfortunately the copy that came to me was rather foxed: the paper was somewhat cheap, and brown blotches have come to disfigure it. I had intended to scan it in monochrome, at 400 dpi. But the foxing meant that such areas would only be black blotches in monochrome. Reluctantly, therefore, I did a colour scan at 300 dpi. The book itself is 724 pages, which makes a mighty PDF of some 200 Mb. Oh well.
I’m uploading the book to Archive.org. I’ll add a link when it completes.
UPDATE: The volume is here: http://www.archive.org/details/AdolfHarnack.MarcionDasEvangeliumVomFremdenGott
I will look at whether I can create a monochrome version, which would be smaller.