All cheats are purchased from the Cheat Ninja, Heidi Shadows. She only appears in certain places at certain times, and for a total of only five hours a day. All but one of the locations can be reached only once you're well into Mission 4. The only one reachable before that(beggining of mission) can be unlocked only by establishing a GBA link to another GBA.Heidi's Schedule is:

1 a.m.-2 a.m.how:Use the time machine to travel to Nov. 29, 1984. This Destination is availible after finishing Level 5
2 a.m.-3 a.m.how:Go to the Carnival
4 a.m.-5 a.m.how:Go to Paradise Island
6 a.m.-7 a.m.how:Secret tunnel between Jail and Dark Tree
10 a.m.-11 a.m.how:Moonbase Zeta
Heidi is a self-contained shop for buying cheats:
CHEAT SHOP INVENTORY:
Rosebud=$2,222(thats how much it gives you)
Silver-Tongue Berry Smoothie=1 charisma point
Gourmet Berry Smoothie=1 cooking point
Clock Berry Smoothie=1 mechanical point
Buff Berry Smoothie=1 body point
Mind Berry Smoothie=1 logic point
DaVinci Berry Smoothie=1 creative point
Slip Of Paper=Password for Club Xizzle and recipe for Strawberry Tiramisu
The Pssword for club Xizzle is "bucket"