If graphic novels are okay, William Messner-Loebs & Sam Keith's Epicurus the Sage
is brilliant and hilarious.
It's got the aforementioned Epicurus, and also Aristotle and Alexander the Great, and sends up all the philosophical schools and the classic Greek legends as well. The scenes where the Sophists "explain" their method of argument and where Epicurus scares the mathematicians by yelling irrational numbers to disrupt their platonic purity or whatever it was are worth the price of admission, IMHO.
Barbara Hambly's ancient Roman historical detective novel Search the Seven Hills/The Quirinal Hill Affair
has as its amateur sleuth a philosophy student, and there's brief but good discussion of Stoicism and other philosophies in the course of the search for truth (and whodunnit). Disclaimer, this is one of my personal favourite novels from one of my personal favourite authors, but I think it's very well done nonetheless and cheapest at Kobo when using the discount coupons.
Margaret Doody's done an entire series with Aristotle as amateur sleuth. I haven't read these but one day intend to.
If you think you might be into historical detective series, Ruth Downie's got a good one set in Roman Britain with a medical doctor attached to one of the legions. But then you said non-military (though Ruso the medicus spends rather little time with the actual military and rather more wandering around Britain and Gaul and crimesolving).
If you want a time-travel piece, Harry Turtledove and Judith Tarr teamed up to do Household Gods
, with a modern woman doing the Quantum Leap-style mindswap with an ancient Roman tavernkeeper and examines the ensuing cultural differences and such. Not much philosophy in that one, as I recall, but an interesting read.