How do you maintain a car if the owner’s manual was lost or missing? Could you try to find and buy the exact same copy for that car online?
For most cars, you can download an electronic copy of the owner's manual, for example, toyota.com allows me to download a file that is the owner's manual for my Toyota - I've done that, my last car didn't come with a user manual.
Buy a Haynes manual . They are worth every penny. Or if you're more advanced, an Alldata subscription.
Owner's manuals are free.
Most manufacturers websites have links to digital copies that you could download to you computer.
You can find the PDF online and usually also through the manufacturers web site.
As others have said, most manuals are online these days. Just Google it and see if it comes up.
Most car manufacturers websites have PDF files of owners manuals on their owners page. You can also go to the dealership and ask for Maintenance information or a replacement manual.
There is also likely to be just a chart for the maintenance schedule too. Just google your year, car, and 'maintenance schedule' and see what you come up with. That really is mainly what you need I would think.