Subsonics

Ever since I got my first handgun pistol in 2017, I've been using 115gr ammo because it's the most available and generally the cheapest. I am now planning on getting a suppressor for my new AR9 pistol, and I understand that using subsonic rounds is best here, which 115 is not.

I don't know yet what suppressor I'm getting. Budget for this will be dependent on what my income tax refund is like this year; I have already seen that most suppressors in the stores tend to cost the same or more than the gun I just bought to host it.

I'd be wanting subsonic both for range ammo and self-defense rounds. What am I looking at here? Do I just need to get a heavier bullet? Or is there something else I need to look at? Not all ammo has the speed charts on the boxes.