Usually, you can find wiring diagrams for your lawn tractor in the operator's manual that comes with the machine. If you don't have the manual, you can try to find the wiring schematic on the manufacturer's official website. You can also try to contact the manufacturer and request the wiring diagram from the company if it's not available on the website. Lastly, there are several websites that have wiring schematics for numerous manufacturers and models available online or upon request.
In order to find a lawn tractor wiring diagram, you need to know its serial and model numbers. You can find them on the identification tag on your machine.
If you need a wiring diagram for the engine, you have to go to the engine manufacturer's website. You can find the engine's model and serial number somewhere on it. Most manufacturers' websites have guidelines on how to find the machine's identification information as well as the format of the serial and model numbers.
When searching for a wiring diagram, you may find a Professional Service Manual for your machine. It should be used only by a trained mechanic; you should not try to repair your machine using the Professional Manual unless you have specific education.