Go to Jag's store (a general store opposite the Holiday Inn at the back of the Bullring) - they currently have:
Cadbury's Mr Big (usually only available in the US and Canada I believe); Nestle Nuts; Hershey's Cookies and Chocolate; Reese's Nutrageous and Peanut Butter Cups (available in Woolies and department store food halls, but not normally anywhere else).
Just goes to show, the things you find when you're not looking for them - I'd gone in for the Mirror; I only looked at their choc to see if they'd got orange Rolos!