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.