Was at the Jasper overflow (can't link gMaps -- just NW of the village).
TBH, in my experience you can always find a place. I've very often slept in rest areas (technically not allowed), picnic spots, etc. I've been turned away only once (at a caravan site, while hiking, under heavy rain...). So I wouldn't worry too much. Perhaps remember that you are in bear country. Kevlar bag or canister.
Have fun -- great trip!