I have plenty websites where you can buy tickets for Camila Cabello concert but, I will suggest the best one among all it's eTickets. I usually buy tickets from this website and according to me, it is the best place to buy tickets not only for Camila Cabello concert but for other concerts too. I live in Canada and I was looking for her concert tickets too.
The quicker you do it, the bigger your chances to get the best seat are. I would monitor special ticket services to be one of the first buyers, here https://mytickettracker.pissedconsumer.com/customer-service.html must be the most helpful one in your case. Please remember that sometimes tickets are being sold out during the first hour if the artist is demanded and popular.