You can refer to this page to find out about test center locations and dates. Note that although there are three test dates per year, not every center is open for every test date
Specifically, you can refer to this PDF (linked to on the above page) to see the test centers offering tests in various places: https://www.ets.org/s/gre/pdf/gre_pbt_center_lists.pdf
It seems like the only location in Ontario offering the Sept 2017 test is Hamilton (McMaster). It seems like there should be Boston centers still open though.
However, since the "early" deadline for Sept 2017 test reg is August 4 (two days from this writing), it might all be full already. I would first check if you can view the Hamilton, ON test option when you try to register online. If so, then maybe the option disappears online when the center is full.
If it's full, then you could do two things.
1) Call ETS directly. Maybe they have closed online reg but last minute cancellations open up spots when you try to call.
2) Standby testing. See https://www.ets.org/gre/subject/registe ... angements/
It will cost you 50 USD more (as the instructions say, bring payment to cover both registration and the 50 USD standby fee). When I last took the Physics GRE, it was in Hamilton as well, and there was a huge line of people testing standby. Maybe 20 people. They all got in. I am not sure what payment method should be used in Canada though, since the fee is in USD. You could call ETS to confirm what to do. Would be a good idea to bring a blank cheque, a credit card, and some cash, just in case!