In Canada, in National Parks, if you're there at the height of the season, you should have booked already.

However, outside National Parks, you might have reasonable luck finding a place if you look for municipal campgrounds. They're the ones run by the towns.

For example, this is Golden's Municipal Campground: Golden Municipal Campground and RV Park - Amenities

Golden is in a scenic area just outside the famous National Parks in the Rockies. The rates for the campground are normal for that area, and it looks like they still have room even mid-season.