What disenchantable item has fortify enchanting

It sounds like your Enchanting skill is too low. Under the Enchanting skill tree, the first perk is called Enchanter. As you spend perk points in Enchanter, it will increase the power of your new enchantments. To get your new enchantments to double in strength, you'll first need to get your enchanting up to 80, spending perk points on the way. Once you have Enchanter at 5/5, your new enchantments will be twice as strong. For example, when I first placed Fortify Archery on an item, it would only fortify it by 8%. Now, and I'm only at 4/5 Enchanter (80% stronger enchantments), when I do the same enchant, it fortifies by 32%.

In addition to Enchanter, you can spend perks in other Enchanting skills such as "Insightful Enchanter" which will increase Skill enchantments on armor by an addition 25%. There are others that will boost your fire/frost/storm enchantments, as well as health/magicka/stamina boosting enchantments, by 25%. These do stack with enchanter for a total of 125% stronger enchantments when Enchanter is 5/5 and the necessary perks are acquired.

When I was searching for an enchant I needed, I did the following.

1. Check Riverwood blacksmith for enchantment.
2. Fast travel to Whiterun Stables, rest until afternoon when the Kajiit vendor is outside the walls, and check for the enchantment.
3. Fast travel to Whiterun and check the blacksmith for the enchantment.
4. Fast travel to each Imperial/Stormcloak camp I had access to and checked their blacksmiths for the enchantment.
5. Fast travel back to Riverwood. Wait (rest) for two 24-hour periods.
6. 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.