I did the same thing and finally found one in a random location after a few hours of questing, I used the wrong water making my potion, aside from that your best bet is to abandon the quest and restart it, I didn't have any luck with the guilds finding one to buy. Be careful posting about buying stuff on the forums though, they don't like that. Google abandoning a quest if you're unsure how, you should see the option in your journal though I believe.