I've did some quest around Riften, and some of the bounty quests for the steward but the option to buy never comes up.

You need to talk to an Argonian woman named Wujeeta about Skooma Trade and then stop it.

You can find here during bussines hours in the fishing docks

or in the evening and night at Haelga's bunkhouse.

Or doing skooma when no one is looking.