You can find Flimm, the "sales freak", in Flipside, on the other side of the water in basement 1, or in the Pit of 100 Trials in Flipside or Flopside. You can find Flamm the "map chap" on the other side of the water in basement 1 of Flopside.
If you go to Flimm on basement 1 of Flipside You will find him selling things such as rare Cards and rare drops from enemies (Such as big egg or fresh veggie). You can also occasionally find him standing next to the door on "Checkpoint floors" of the Pit of 100 trials, where he will be selling mostly Items that would be useful in the Pit of 100 trials (Such as Shroom Shakes and Thunder Rages).
Flimm has better prices for most things than other places when we find him on basement 1, but he has normal prices for things if we find him in the Pit of 100 trials.