Are you looking for one available electronically, or just the editors/publishers of the accepted, "authoritative" print editions?
I'll get you the latter information a little later tonight, if no one else has yet: in the mean time, if you're just looking to read a little, here is a link to a decent English translation of his Romans lectures: Romans.
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Also, for what it's worth, Aquinas' exegetical work is fascinating, whether we agree with his conclusions or not.
As far as I know, the Leonine edition of the Pauline lectures is yet to be published: it will be vols. 32-25. This should become the standard edition.
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Just found this: Here is a link to Corpus Thomisticum, of the Fundación Tomás de Aquino. Do a page search for "In Epistolas S. Pauli", and it will take you to, obviously, his Pauline commentaries, the first of which will be Romans. It's in Latin, and it's the text for which I gave bibliographic information above, which, according to the same foundation, is the "best" edition.