Try at the Garbanzo Bike and Bean or Garbanzo Kids located at the base of Whistler Mountain or Can Ski Marketplace when they change over to the summer products. Small armor can be hard to find but the selection is improving. In the past we have recommended using adult arm pads for leg pads, BMX or skateboard pads are an option for small arms.
The DFX Mountain Bike Camps Parents Info Page on Facebook is also a great place to pick up used equipment.