If you are talking about the Jet Scooter, then it is on the beach. You should hear about it in the dungeon for non-combustible garbage in the third level of the basement. Don't use the things Motabian gives you. Only use the Cream Bar if he gives you that.
Wow, I haven't played Phantasy Star II sence it came out in 1990 for the Genesis. I have it for the PS2 too, with Phantasy Star III, and IV on the disc. I got it just to play those games. I still have the game and the Genesis. I still have the little Hint Book for the game, that you had to buy at the book store.