Check out freecampsites.net and Boondockerswelcome.com. We rarely pay for any CG's and never book in advance but that's just us. We have on occasion stayed in a Church carpark, restaurant carpark on a river after asking the host, overnight in the parking area of an attraction we intend to view the next day.
Check out the Crownland possibilities of the provinces you intend to visit. In the west we check Public Land Use Zones for true boon docking, and found some awesome locations to call home for a few days to a week typically for us besides water for kayaking/rafting.
There are tons of options if you dig around a bit. Some cities have ordinances that don't allow but check for current and up to date info, because some brought those rules in and local businesses due to losses because of that managed to get it changed back again to become RVer friendly again. Do check current info not dated info in your research.
Maritimes are beautiful, we've done all across NS, PEI, NB, our next trip out East in the future, we intend to hit NFL and Lab and do the ferries, and we will be boon docking our way at least 9 out of 10 nights as is our norm.
Edited when realized this was provinces not states.