Xbox – find some smithing potions in Skyrim

The giant golden globe roughly in the centre of Blackreach is not the lexicon machine, but it provides a useful point of reference to find it. While standing below the giant golden globe, walk directly south. As you pass through the southern gate of the wall, you will be on top of a set of stairs. Slightly to your right is a tall tower that connects to the ceiling. This is the Tower of Mzark. To reach it, you will cross a bridge over rapids which flow to the edge of a waterfall. When you reach the tower, take the elevator to the tower's interior, solve the puzzle, and you will be able to transcribe the Elder Scroll into the lexicon and then retrieve the Elder Scroll from the machine.

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.