I had a quick look and here on itch; I found WhiteNightStudios, who has a few free and paid 8 directional assets. There's also some assets on OpenGameArt.org, but I'm guessing that you'll have to read through what licensing each asset uses (I dunno; never used the site).
It's interesting that there aren't many asset packs... I'm guessing it might be because when you make a retro FPS and mix asset packs; they can easily clash since changes in the pixel art style, colour usage and resolution become very obvious in this kind of game.
Just out of interest, what kind of style, theme, resolution and so on, are you looking for?