There are 2 ways to get the Armour, Bunny Suit for Jessica in Dragon Quest VIII: Journey of the Cursed King.
The first way is as a prize. You can get this item when you open up the monster arena on the road to Pickham. You need to win Rank F to win this prize.
For an indepth strategy on how to beat Rank F, and get through the whole Monster Arena, go here:
The second way is through alchemy. If you combine the Silk Bustier (that you can buy for 5,500 gold at the Argonia bazaar) with a Bunny Tail accessory (also at the Argonia bazaar for 50 gold, or found in the Forest west of Simpleton in a chest, in Pickham inside a cupboard, in Baccarat inside a dresser, in Arcadia inside a cupboard or Dropped by some monsters very rarely.)
To make Jessica's apperance change, you will need to equip her with the Bunny Suit as her armor, and equip her with the "fishnet stockings" and the "bunny ears".
You need to make the Bunny Ears by mixing a Bunny Tail and a Hairband in the Alchemy Pot. You can get a hairband as the starting headwear for Jessica, or buy it for 150 gold from a man in Peregrin Quay, which is actually the item shop.
The Fishenet Stockings are the prize given for 28 Mini Medals traded at the Castle, or dropped by Ice Queens.
For more informantion on where to find the Mini Medals, and where to trade them, go here:
Good Luck.Source(s): Beat Dragon Quest VIII
Used this guide as well:
Dragon Quest Jessica
You have to unlock the Coliseum. Beat the F rank to get the Bunny Suit.
Wear the bunny suit, net tights (obtained by obtaining 28 or more Tiny Medal) and Bunny Ear Band (combine Hair Band and Rabbit's Foot).