If you're on the hunt for crossover stories featuring 'Goblin Slayer', start with the big fanfiction hubs — they’re where most people post and curate crossovers. Archive of Our Own (AO3) is probably the best place to begin; its tagging system is terrific for crossovers, so you can search the 'Crossover' tag and then filter by additional fandoms like 'Dark Souls', 'The Witcher', 'Dungeons & Dragons', 'Skyrim' or 'Harry Potter'. Use the search bar with queries like "Goblin Slayer crossover" or the site-specific Google search: site:archiveofourown.org "Goblin Slayer" "crossover". AO3 lets you sort by kudos, hits, and bookmarks, so you can find well-loved fics quickly.
FanFiction.net and Wattpad still host lots of crossover work too, though their tagging and search are clunkier. On FanFiction.net try genre and character filters, and on Wattpad use the hashtag search (e.g., #GoblinSlayerCrossover). Pixiv's novel section and Tumblr are great for short crossover one-shots and drabbles; on Tumblr, search the 'Goblin Slayer crossover' tag and follow reblog chains to discover authors. Don’t forget Reddit — subreddits like r/GoblinSlayer and r/FanFiction often have recommendation threads or pinned lists with crossovers.
A few extra tips: check author series pages and bookmarks (favorite authors often write multiple crossovers), pay attention to content warnings and ratings (some crossovers can be grim), and use browser bookmarks or AO3 subscription features to follow writers. Personally, I love finding a surprising pairing like 'Goblin Slayer' x 'Skyrim' — the grimdark practicalities line up so well — and the hunt is half the fun.