Specifically oldest manuscripts. Is there a website that has scans of the primary sources? I’m using Cassius Dio as an example. I would be curious to know about other sources (Herodotus, Thucydides, Caesar’s Gallic Wars, or whatever).
The Dead Sea Scrolls and the Nag Hammadi sources are things I would call genuine primary sources. I’m interested in analogues to those.
There are two ways to find out this kind of information:
Consult the New Pauly Suipplements vol. 2, Dictionary of Greek and Latin authors and texts, which contains quick summaries of the principal manuscripts, editions, and translations of pretty much every Greco-Roman author. These summaries are very informative but also very far from complete.
Consult the standard critical edition, which will contain a detailed and thorough account of the manuscripts.
In this case the standard critical edition is that of Boissevain (1895-1931), published by Weidmann. Pages ii to lxxxix of the introduction (pp. 2 to 89) give a detailed account of the surviving manuscripts, excerpts and summaries, and fragments.
In some cases, the libraries where these manuscripts are held do make scans available. But manuscripts aren't being scanned at every library that holds manuscripts, and the ones that are scanning are going to take a long time over it because it's an expensive process, and no one wants to pay for it.
There are a few major collections that are exceptions, and are very diligent about scanning their collections and putting them online for free: such as the Biblioteca Apostolica at the Vatican, the various state libraries in Germany, and the Bibliothèque Nationale de France. (Plenty of other libraries are working at this too, they've just got some catching up to do.)
There are far too many manuscripts to repeat Boissevain's account here. The New Pauly sums up the most important manuscripts as follows:
| Callmark | Date | Contents |
|---|---|---|
| Vaticanus gr. 1288 | 5th/6th cent. | bks. 78,2,2–79,83 |
| Laurentianus gr. 70,8 | 11th cent. | bks. 36,18,1–50,6,2 |
| Marcianus gr. 395 | 11th cent. | bks. 44,35,4–60,28,3 |
| Parisinus 1397 A | 11th cent. | frgs. 57, 53–60; 63–71; 81; 83–86; 58,1–6 |
| Vaticanus gr. 144 | 5 July 1439 | bks. 36–54 |
Of these, you may well be able to find scans of the Vatican, Laurenziana, and Paris manuscripts on those libraries' websites. Here, for example, is Vaticanus gr. 1288 -- though be advised it's in pretty poor condition (unsurprising for a manuscript that old: surviving manuscripts of ancient works aren't usually older than the 9th century).