Where can I find the driver for my cannon mx870 printer

If the Canon is connected to your local Network, then you should be able to use AirPrint from the new iMac without any additional drivers. see > Add a printer on Mac - Apple Support

For additional Canon Support, go to > https://www.usa.canon.com/support/p/pixma-mx870

In the alternative, you will need to check Cannon's web site for a compatable printer driver.