Sometimes the hunt for rare Sebastian pieces feels like treasure hunting and I actually enjoy the chase. If you mean Sebastian Michaelis from 'Black Butler', my top starting points are Pixiv and BOOTH.jp — Pixiv is where many Japanese artists post their best, often with exclusive illustrations or links to prints on BOOTH. Search both English and Japanese terms like "Sebastian Michaelis" and "セバスチャン 黒執事" to dig deeper. I bookmark artists and use Pixiv's tag-follow or bookmark features so I get notifications when they post rarities.
Tumblr, Twitter (now X), and Instagram still hide gems if you chase old tags and artist archives. For deleted or hard-to-find posts, the Wayback Machine and Tumblr archives can resurrect stuff long gone. For physical rarities, Mandarake, Yahoo! Japan Auctions, and Melonbooks are lifesavers — I use proxy services like Buyee to snag prints and zines from Japanese sellers. Etsy and eBay sometimes carry rare convention prints too, but check artist credits carefully.
I also recommend booru sites (Danbooru/Gelbooru) for tag-based searching, and niche Discord groups or fan-run tumblrs where collectors trade scans or point to obscure circles. Always try to support artists — buy prints, commission them, or follow their Patreon/Pixiv FANBOX — it keeps rare work coming. Hunting through these places has netted me pieces I never thought I'd find, and it feels awesome when you finally score something unique.