I need to disable my main keyboard as one letter is not working and it's out of warranty

Solution 1:

You can disable the keyboard by unloading the kernel extension with

sudo kextunload /System/Library/Extensions/AppleUSBTopCase.kext/Contents/PlugIns/AppleUSBTCKeyboard.kext/

To reload,

sudo kextload /System/Library/Extensions/AppleUSBTopCase.kext/Contents/PlugIns/AppleUSBTCKeyboard.kext/

You didn't say what version of macOS you are using, I believe this syntax works as of 10.10. Some tweaking may be needed depending on your version.

Solution 2:

Actually you can buy replacement keyboards online, at the very least eBay has a number of them on sale. Have a look at ifixit.com as they show how to do it and it should be able to give you an idea if you will be able to replace it yourself.

The insides of MacBooks can be a bit fiddly but with a little care and the right tools (which ifixit sells) you can probably do it yourself.