Truck stops, designated rest areas along the expressway, public parks and beaches, and usually carpool lots are all fine. I'm American, but Canada should be no different.
Check local and regional trespassing and loitering laws. In America, you generally can't get in trouble unless you ignore a sign or an order to leave, or you return after receiving a legal warning.
As such, any publicly accessible parking lot is fine until you're told otherwise. Large, full parking lots like a shopping center work well. It's not unheard of for people to take a nap on a lunch break or while waiting for a family member to finish shopping.
Be prepared to explain yourself. Even if you're doing nothing illegal, failing to dispel suspicion can still be inconvenient for you. To avoid causing alarm, don't frequent the same private property. Instead, rotate between multiple locations so you don't develop a noticeable pattern.
Finally, get some jumper cables. If you're going to sleep in your car, you will eventually make a mistake and leave something turned on, so plan for that now. Good luck and I hope your situation improves soon.