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.
gaming.stackexchange.com/users/15536/Daywatch
If you speak to Frida the alchemist in Dawnstar, she may give you a quest to retrieve the Ring of Pure Mixtures, which gives a bonus to Alchemy.
When you return the ring to her, you can pickpocket it back and disenchant it to learn Fortify Alchemy.
Alternatively, you may eventually find one in a dungeon or by pickpocketing an alchemist or mage.
gaming.stackexchange.com/users/3454/Joe Dovahkiin
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.
gaming.stackexchange.com/users/14585/Goblinlord
If you have the coin you should be able to find the Circlet of Minor Alchemy (a 12% boost) at Sadri's Used Wares in Windhelm, and both the Circlet of Major Alchemy (a 17% boost) and the Circlet of Eminent Alchemy (a 20% boost) at Radiant Raiments in Solitude.
If they're not in stock, just wait 48 hours or reload your game and they should show up.
A chest in the cave at the Halted Stream camp has the Hide Bracers of Alchemy which gives you a 15% boost.
If you kill Krosis at Shearpoint you can loot his mask from his body.
His mask will give lockpicking, archery, and alchemy a 20% boost.
The Ring of Pure Mixtures is in a chest in the Forsaken Cave and will give you a 12% boost.
Muri's Ring has a boost of 15% and you get it from Muri as a reward for doing the optional objective in the Mourning Never Comes Dark Brotherhood quest.
There are a few other objects you can find to boost alchemy, but they found by looting random chests once you reach various levels.
gaming.stackexchange.com/users/18965/System Folder
In terms of unique enchanted items, there aren't any.
You'll need to either find a random enchanted item in a chest/on an enemy or try and find one in a general store/blacksmith.
You don't need a good piece of gear to learn the enchantment, just something with the enchantment on it.
gaming.stackexchange.com/users/15471/Tarkenfire
I noticed that the vendors do not carry the exact same items when loading a savegame.
This worked against me actually once as I expected to be able to buy something they had in stock after loading a savegame.
However if you want to take advantage of it, I noticed as well that after a couple reloads the merchant's inventory
no longer changes.
So you might try reload a couple times, see what comes up.
Conversely, if you see something interesting and you want to take a break, consider buying it before quitting the game as it may not be there when you reload your save.
gaming.stackexchange.com/users/1657/Bleep Bloop
I've been reading all sorts of things about people "gamebreaking" and crafting ultimate everythings. I wanted to join in, but I can't seem to find an item to disenchant so as to get the Fortify Alchemy enchantment (that I would use to craft fortify enchantment and so on...).  Does anyone know where I can find what I'm looking for?
Where can I find an item I can disenchant to learn Fortify Alchemy?
gaming.stackexchange.com
<the-elder-scrolls-5-skyrim>
gaming.stackexchange.com/questions/38893
gaming.stackexchange.com/users/15181/Guy0203
Check for enchantment at Riverwood blacksmith or Whiterun stables. You can also check at a vendor or look for a random enchanted item in a chest.  It seems that in order to find the proper item and disenchant it, RNG has to be taken into consideration.
The community is proposing few methods to search for the enchant in question.