Welcome to the forum and Nanaimo, soon.
There are some big stores at the malls you mention, and also at Country Club Mall. There are a few more stores, too, all around Woodgrove.
There are certainly cheap deals at Boo's Streetware, downtown on the highway and Commercial.
Other than that, I'd recommend the thrift stores.
What are trainers? I always wondered in Harry Potter, too. Sounds like shoes.