I second that,
In printed form, that’d be Apologia I, 1xv-1xvi (what ever that means). He says things like: “This food is called Eucharist with us, and only those are allowed to partake who believe int the truth of our teaching…” and “…the food…is…the flesh and blood of Jesus who was made flesh.” Justin has a fondness for saying things like “we believe” and making little side-notes in the midst of his sentences, so I used a lot of ellipses.
I got this out of “The Early Christian Fathers,” Henry Bettenson, Oxford: 1956. But there are numerous books on the Fathers–Justin whom I find most intriguing and most inspiring.
However, as a fairly concise, easy-to-read introduction, I’d recommend an Evangelical based publication called “Shepard’s Notes Christian Classics: The Writings of Justin Martyr.” It’s sort of a small ‘study-guide’ which actually packs a lot of good info. Justin’s belief in the Eucharist is not glossed over, but expounded upon.