In Sniper Elite 4 you only had one rifle that could be suppressed, but in Sniper Elite 5 it seems every sniper rifle can have a suppressor. Here’s how you get your first one.
You’ll get it during the second mission via the rifle workbench. This workbench is located in the basement of the chateau that you’ll need to enter in order to complete your main objective; there is no “best” way to enter, and all can be approached fairly stealthily (although a direct frontal assault is not recommended).
I personally approached from the north-west tower of the chateau and just lured enemies into traps to thin the herd after taking out the alarm (which is inside the courtyard); you can take this alarm out easily from the garden in the upper left. of the blue circle. I then just went down the stairs near where the workbench icon is from that same courtyard (you can’t miss them).
You can also reach it via the eastern moat; there are a few ways to get down into the moat, so take your pick as to how you get there.
Once inside, you’ll find the workbench among at least a couple standard soldiers that are easily dealt with.
This workbench will unlock the Maxin 1910 suppressor and the Moore-1 suppressor for sniper rifles.
That’s it – happy sniping, ghosts!