That store is
Polar Battery at 1258 Boundary (Burnaby side, between Charles and William streets). Take your old one along so they can match it, in case it needs to be a particular size/dimension to fit.
You can also find those batteries at
RP Electronics just near Brentwood Mall. 12V/4.5Ah should go for around $20-$25.
Of course, I just go across the highway and get them wholesale for about $8
