The most straightforward spot is Archive of Our Own. Its tagging system is honestly life-changing for niche pairings like this—you can filter by character, relationship, and any specific tropes you're craving. I usually sort by kudos to see what the community has collectively loved. Wattpad has a different vibe; the stuff there can feel more raw or meme-influenced, which sometimes fits the Postal Dude's chaotic energy perfectly. I stumbled across a few surprisingly introspective ones there that framed his nihilism in a weirdly romantic light. Tumblr is trickier to search, but some authors post snippets or links there, and the reblog chains can lead you to hidden gems you'd never find through a normal search engine. The fandom isn't massive, so it's worth checking all three; the characterization can swing wildly from unhinged violent comedy to surprisingly poignant melancholy depending on the platform.
A word of caution, though: the 'popular' ones aren't always the best-written. Some of the highest-kudosed fics lean heavily into the shock humor from the games, which is fun but gets repetitive. I've found more interesting takes by filtering by 'completed' and then sorting by date updated. The quieter, character-study pieces tend to get buried. There's this one author on AO3 who writes him with this terrifying, quiet vacancy instead of loud rage, and those stories stick with me way more than the ones where he's just blowing stuff up with the reader as a sidekick. It's a pairing that can support a lot more depth than it gets credit for.