As Fallsview mentioned in a previous comment, the large parking lot on Clark Ave appears to be run (and probably owned) by the Fallsview Casino.
I’ve never seen more than 1 or 2 cars in the lot at the same time. There doesn’t seem to be a way to pay going in, so perhaps it is only for card holders of some kind. I’ve emailed their “concierge” asking if I can park there. We’ll see what they say…