seeking lesser known roman letters of correspondence

Hello, first time in this forum :)

I've been reading letters of cicero and pliny, and I just found out there are extant letters of augustus which I never knew existed. I was wondering if someone could direct me to where I might be able to find publications of letters from other people. I would like to read all extant letters from ancient rome, if possible. Is there a compilation I can buy, or a site that lists all current translations? I'm proficient only in english. I'm desperate to find these other letters.

Thank you so much for your help,

Agro