You might try Hock Choon's Grocery next to Lanson Place Hotel 1 Jalan Ampang Hilir. They carry a lot of American foods there. Freepmail me your phone number.
Ampang Mini Mart was a no go. The Chinese guy who runs the place is overseas at the moment and so he left his aged father to run the shop. The old man there is always grouchy and never knows anything. He seems pissed off that they are still sitting on a mountain of Ocean Spray Cranberry juice that they have been trying to unload for the last 6 months. I didn't notice any Diet Dr. Pepper again and so I asked him if he had any and he mumbled something and growled at me.
Bleh... I paid 3 bucks for some Cherry Twizzlers and left.