Where can I find an item I can disenchant to learn Fortify Alchemy?
When I was searching for an enchant I needed, I did the following.
- Check Riverwood blacksmith for enchantment.
- Fast travel to Whiterun Stables, rest until afternoon when the Kajiit vendor is outside the walls, and check for the enchantment.
- Fast travel to Whiterun and check the blacksmith for the enchantment.
- Fast travel to each Imperial/Stormcloak camp I had access to and checked their blacksmiths for the enchantment.
- Fast travel back to Riverwood. Wait (rest) for two 24-hour periods.
- Repeat the whole process as necessary.
Alternatively, you could sit at one vendor, check what they are selling, then just wait two 24-hour periods and check again. Sometimes it can take a while.
I think some enchantments might not show up until you reach a certain level. I have a level 41 and he has not seen the fiery soul trap enchantment yet.
Depending on the enchant, make sure you have a couple thousand septim or you might end up doing all the waiting and finding you don't have the cash to get the item with the enchant you desire.
For the enchant you specifically mention... you might want to try the quest Jonathan Drain mentioned in his post.
If you complete the optional objective in the Dark Brotherhood quest Mourning Never Comes you get Muiri's ring which has Fortify Alchemy 15%, I believe. That's the only unique item I can think of off-hand.