The Vatican Library has digitized and publicly shared over 15,000 of its own manuscripts on its website: https://digi.vatlib.it
After a quick search for “Aquinas” this amateur researcher was able to spot manuscripts originating as early as the fifteenth century from St. Thomas’s commentaries on Aristotle, the Summa contra gentiles, De potentia Dei, De malo, the Scriptum, Quodlibeta, and the Summa theologiae, to name a few. Pages and pages of results turn up. Have fun exploring!
With thanks to Aleteia.org for originally sharing the story.