There are
Walmart locations all over B.C.
I have found ponchos at Walmart. Specifically at the New West. location, in the girls' clothing dept. area. It's usually near where they keep little kids' umbrellas. I have found adult sizes there too.
I have also found ponchos at Zellers, near the purses, jewellry, makeup sections. Specifically in Richmond Lansdowne Mall location.
I just buy the 99cents ones, nothing too pricey. That way, if you don't want to carry them around the parks anymore, you can just dispose them or give them away.
Zellers has some that are of thicker quality and are a bit more expensive; but they also carry the cheaper 99cents ones.
Hope that helps!