Ontario is bigger than most European countries and as such the Bills cannot claim it exclusively for their own.
That said, the Bills can reasonably claim the Niagara region, St Catherine's and Hamilton.
There are too many bandwagon fans in Ottawa and Toronto for the Bills to claim these cities exclusively.
Ottawa is composed of many ex-Montrealers who fled separatist Quebec with Patriot loyalties. Ottawa and Montreal still have loyal CFL fanbases.
Still Bills fans loom large in both Ottawa and Toronto.
As you go west of Niagara Falls to Kitchener-Waterloo, London and Windsor you are in Detroit Lions Territory. Everything from Sault St Marie to Manitoba is split between the Packers and Vikings.
This does not account for fans of "national" teams like Dallas, Pittsburgh and the Raiders for which there are strong followings.
I have only met one Jets fan in nearly 40 years of living in Ontario.