If you are travelling the Banff to Jasper Corridor, we've overnighted at Brewsters (icefields), and there are other pull ins along that stretch. Hard to find actual boondocking "in" Jasper itself nowadays. Think IIRC Gonewiththewynns.com found a great little place to dry camp overnight on their way up to Alaska from Banff to Jasper etc.
When we used to Ski most weekends, we'd also overnight in our MH & 5er prior at Sunshine Village (between Banff & Lake Louise and very beautiful area) They'd let us stay in the Staff area parking due to our size. Also Golden & Revelstoke we've boondocked at off the TransCanada.
We never book anything unless it's an absolute necessity and we know we want to spend more than two days/nights in the area, such as San Fran, L.A. Keys Fla and NYC. Mainly we wing it and we've never not found somewhere safe and comfortable to stay overnight.
Of course we appreciate everyone has different comfort levels, and our tolerance for flying by the seat of our pants, has grown for sheer panic when we started boon docking 17 years ago, to now knowing, there'll always be something, somewhere. Police can't force you on when you are tired to cause an accident, but they in turn might guide you to a safer place to overnight when asked.
FTW.
It Takes No More Effort To Aim High Than To Aim Low - Reach For The Stars