Here is all the locations (chronological order)/items/cost for each encounter with Stiltzkin the world traveling moogle, he wears a yellow do-rag and yellow satchel to make him stand out from other moogles.
He will not sell you items if you have 99 of an item he is selling. All of these can be missed.
1. Burmecia/Vault, after saving the injured Burmecian from the falling statue and advancing two screens forward you will be in the palace square with a door on the left and another one the right. Enter the right door and interact with the chest in behind the save moogle. Stiltzkin enters and is selling a soft/hi-pot/ether for 333 gil
2. Cleyra Settlement/Sandpit, at the beginning of the assault on Cleyra you will have a forced encounter, afterward go right (where you fought the AntLion) and not upstairs, you find a save moogle and Stiltzkin selling a ether/hi-pot/phoenix pinion for 444 gil.
3. Fossil Roo, virtually unmissable, next to the save moogle and sells remedy/ether/phoenix pinion for 555 gil
4. Conde Petite Sanctuary Path, again virtually unmissable, next to the save moogle and sells a tent/magic tag/ether for 666 gil.
5. Alexandria/Steeple, after the events at the Iifa Tree, you will regain control of Vivi in Alexandria, go to where you meet the first moogle in the game and you'll find Stiltzkin there selling a phoenix pinion/hi-pot/elixir for 777 gil.
6. Oeilivert/Star display, after the small scene with Zidane and co. entering the huge door, immediately take a right into a screen with a save moogle and Stiltzkin selling a hi-pot/emerald/elixir for 888 gil. He will not be there if you finish all the steps to reach the dungeons boss.
7. Bran Bal/Storage, after you gain control of Eiko, head for the save moogle in the village and Stiltzkin shows up selling a diamond/ether/elixir for 2222 gil.
8. Alexandria/Steeple, after obtaining the Invincible, head here for the final encounter with Stiltzkin, he sells a moonstone/ruby/elixir for 4444 gil.