If you already missed out on collecting any of these items, you can do naught but sigh and move on with the rest of the game. The bad thing is that all of these items are tied to transient, one-time quests that, once completed, are no longer available. The good thing is that you'll get these items regardless of whether prior knowledge of this quest existed. In fact, if you bring him these items before the end of Act 1, his shop's inventory will be upgraded afterward. No doubt that these are rare items and dangerous to obtain, too, but the herbalist is a businessman after all and promises to make it worth your while.
Giving you a list that you can complete in Act 1, he asks you to fetch him a Dragon Fang, Spider's Silk Gland and a pure Ironbark. If you accept, this long-running quest will span over several Acts, but Solivitus knows not to be too demanding all at once. Once you've completed the Long Way Home quest, you will be able to find the savvy herbalist, Solivitus, in the Gallows courtyard and start up a business-centric conversation.
