If your car is still in production, then it is easy to get a replacement from the dealership that sold you the car. They will probably have them in the showroom to give to you (maybe with a charge). If your car is no longer in production then they are unlikely to have them in stock but will probably be able to order you one.
There is no definitive website for obtaining free car manuals, but here are a couple you could try that sell rare owner’s manuals and repair manuals: www.factoryautomanuals.com/ and books4cars.com/ However if you perform a search in Google for your car’s make and model then you might get lucky. Another good place for obtaining car manuals is eBay. Again, perform a search for the make and model of your car and see what comes up.