I sometimes need to check the extensive notes in the old edition of Cicero’s letters by Tyrrell and Purser:

Robert Yelverton Tyrrell and Louis Claude Purser, The Correspondence of M. Tullius Cicero, Arranged According to its Chronological Order; with a Revision of the Text, a Commentary, and Introductory Essays (7 vols., Dublin: Hodges & Figgis, 1879-1933)

The volumes are not as easy to find online as they should be. Various libraries have nice sets of organized links to electronic versions, but these direct to HathiTrust, which means that several volumes are challenging or impossible to access outside the US.

Most of the volumes are on archive.org, but they are a pain to search. So, here is an organized set of links for the files. Most are on archive.org, but a couple I could only find on Google Books, meaning there may be limited access in some countries.

First Edition

• Volume I (1879, Google Books) letters from 68-57 BCE
• Volume II (1886) letters from 57-52 BCE
• Volume III (1890) letters from 51-50 BCE
• Volume IV (1894) letters from 49-45 BCE
• Volume V (1897) letters from 45-44 BCE
• Volume VI (1899) letters from 44-43 BCE and letters of uncertain date
• Volume VII (1901) Index

Second Edition

• Volume I (1885, “Second Edition”) letters from 68-57 BCE
• Volume I (1904, “Third Edition”) letters from 68-57 BCE
• Volume II (1906) letters from 57-52 BCE
• Volume III (1914) Google Books letters from 51-50 BCE
• Volume IV (1918) letters from 49-45 BCE
• Volume V (1915) letters from 45-44 BCE
• Volume VI (1933) letters from 44-43 BCE and letters of uncertain date (I don’t see this volume online in the usual places.)