Man, you'd be surprised how much 'Absolutely Fabulous' fanfic is still kicking around out there. It's an older fandom, so the landscape is pretty specific. Archive of Our Own is the absolute center of gravity these days—their tag system makes it super easy to filter for Delia/Patsy (or 'Patdeli,' though I've seen both tags used). You'll find everything from fluffy 1960s AU meet-cutes to post-series angst. The quality there is consistently high, writers are really dedicated.
FanFiction.net has a decent, if aging, archive. It's worth a search, but a lot of the stories are from the early 2000s and the interface is clunky. You might stumble across some real gems from that era, though, pure nostalgia. Tumblr is weirdly vital for this pairing too, more for headcanons, drabbles, and moodboards than full multi-chapter epics, but the community feels very alive there. The reblog chains can lead you to some fantastic short-form writers you might miss on the big archives.