don pianta is behind the the shop in rougeville(rouge cityi dont know...), next to the playing slots...im trying to remember how to get to him...to get a yoshi you need to get up to chapter umm....3, god i havent played that game in a while...its the place where you get the brown crystal star in the sky...but yeah to get yoshi is after getting to don pianta.
wait...i remember now...you buy a dried shroom and dizzy dial(IN THAT ORDER!!) at the shop with the girl boo,she'll then ask you a question...the answer is "yellow". After that she'll let go through a back door.Go up the stairs and there's Don pianta. He says he'll let you have a ticket but after you find his daughter, check the pier(where you first start the game).After talking to her,go back to Don pianta.Tell him you found her. After you get the ticket, go out the shop (meaning outside the actual shop!!)and go toward the blimp/train station turn right and the chhep cheep will let on onto the blimp. You'll get to ride the train later. And yoshi, you have to go through chapter 3(just some of it) and you'll get him!!!