Thomas’s commentary on Peter Lombard’s Sentences is online in searchable PDF format (thanks to the SIEPM site for this information), which you can download via the following four links: 

1. In I Sent. (PDF)
2. In II Sent. (PDF)
3. In III Sent. (PDF)
4. In IV Sent. (PDF)

This is the edition attempted—never fully completed, alas (bk 4 goes only up to distinction 22)—by Pierre Madonnet and Fabien Moos (Paris: 1927-1947). Still, getting this edition for one’s personal library was the pearl of great price when I was in graduate school in the 1980’s.

Note: the file sizes are daunting (~50 MB each).