I still get a little giddy hunting for weird 'Bleach' pieces, and the Soul King design pops up in a surprising variety of merch if you know where to look. For official stuff, think posters, wall scrolls, and clear art prints that use key visuals from the manga/anime — limited-edition Blu-ray or DVD box sets for 'Bleach: Thousand-Year Blood War' sometimes include art cards or booklets featuring the Soul King. Prize goods from lotteries like Ichiban Kuji and Banpresto prize figures also occasionally show up with the Soul King motif, especially around anniversary drops.
On the smaller, more everyday end you'll find acrylic stands, keychains, enamel pins, stickers, phone cases, and tees that rep the Soul King aesthetic. I snagged an acrylic stand at a con and a glossy poster from an online Jump Shop restock; both capture that eerie silhouette really well. If you're after something rare, keep an eye on Mandarake, AmiAmi, and eBay for deeper back-catalog items — the hunt is half the fun, honestly.