Does anyone know of any websites that sell 158 grain 9mm bullets for reloading? I am looking to load up some subsonic rounds.

As far as I know, there never was a 9 M/M (.355") bullet in 158 gr. weight. There are, however .357" bullets in that weight and I don't think the extra .002" in diameter will give you any heartburn with a sub-sonic load. The factories usually load their sub-sonic 9 M/M ammo with a 148 gr. bullet.

I'd just use the 158 gr. round nose bullets made for the .38 Spl. I cast my own handgun bullets, and have found that in sizing them to .357 or .358", I get much better accuracy from my 9MM handguns.
I checked the groove diameters on three 9MM handguns and they were not .355-.356", but one was .358 and the other two .357" Might be one of the reasons 9MMs have such a bad reputation for shooting cast lead. The bullets are too small.
Paul B.