Anyone know a place to comission an artist for a doujin?

Any good sites to commission an artist for a hentai manga or doujin or any well know artists who do commissions? I've been looking around for a while but haven't really been able to find many.

Pixiv, if you contact them lots are open to commissions.

Commissioning an entire doujin from a well known, presumably japanese, artist? Has it ever been done?

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