In Generation IV, plants are purchased from Hikers in the Underground.
Pretty Flowers - 8 to 9 green spheres
Lavish Flowers - 8 to 9 green spheres
Dainty Flowers - 8 to 9 green spheres
Lovely Flowers - 8 to 9 green spheres
Poké Flower - 11, 20 or 30 pale spheres
Bonsai - 8 to 9 green spheres
Potted Plant - 20 to 21 red spheres
Dolls, like many other decorations in this generation, are available by trading spheres to Hikers in the Underground. They can also be purchased in the fourth floor of the Veilstone Department Store. Instead of placing them on desks or mats, dolls, like many other decorations, cannot be placed on top of any other objects.
Mime Jr. Doll
Tools are used to place traps within the secret base. There is one tool for each type of trap in the game, excluding Move Traps, Hurl Traps, Reverse Traps, and Confuse Traps.
Alert Tool 1
Alert Tool 2
Alert Tool 3
Alert Tool 4
Big Smoke Tool
Source: Bulbapedia/ Decorations