If you're looking to squeeze the most noise reduction out of your 9mm Luger suppressor, try this subsonic ammo from Fiocchi.
These cartridges come in a bulk case of 1,000 and fire an extra-heavy 158 grain full metal jacket bullet. The weight of the bullet combined with a standard pressure powder charge results in a low muzzle velocity of 850 feet per second. This is well below the sound barrier and guarantees a quieter shooting experience at the range.
Fiocchi ammo is made in Ozark, Missouri and Lecco, Italy using non-corrosive primers and brass cases.