The medical reports should be submitted with your ITA, along with your police certificates so you don't need to worry about them until you find that your EOI has been selected. They do cost a fair bit so you need to time them so that they are still valid when your ITA package is submitted. Once you have chosen your panel doctor and you think it might be getting near the time your EOI will be selected, perhaps a quick call to them to find out how long you're likely to wait for an appointment might be in order.
As Sparrows says, you can find a panel doctor in your area
here . Perhaps someone will come along with an idea of how much these medicals cost.