Honestly, the heart of 'Zizzy x Willow' stuff right now is probably Wattpad. I know, I know, a lot of people turn their noses up at it, but for this specific ship from the 'Piggy' game fandom, that's where the volume is. It's a younger audience mostly, so you get a flood of high-school AUs, sweet first-kiss scenes, and a lot of hurt/comfort stuff. The quality varies wildly, obviously, but the sheer number of stories means you can filter by reads and comments to find the decent ones.
AO3 is where you go for the more polished, structured pieces. Less quantity, but the authors there tend to be older and more deliberate with tags and tropes. I found a fantastic post-canon survival fic there last month that actually made me care about the game's lore. Tumblr's still good for drabbles, headcanons, and that immediate community chatter, but for full-length stories, Wattpad's the starter hub and AO3 is the archive for the keepers.