Where do I find a codriver / navigator?

The first option is one of your friends in your local area. Seventy percent of rally drivers get to their first rally with a friend who has also never rallied before, so having a team of complete novices is certainly ok.

Another option is to attend some rallies and get to know the teams that are competing there. Swap some emails back and forth at the rally, and then when the time comes for you to enter a rally, get in touch and ask them if they or someone they know is available.

You can make open posts on internet forums, this can sometimes dig up people who were planning to go to the rally anyway, but already have all the safety gear and licenses.

You also have the option of RallySolo, which is entering with no codriver. It's a trade off of weight advantage versus information/teamwork disadvantage. A big adventure for you either way.