Are there perhaps well established venues for being hired as a temp/contractor on existing projects? Some reputable site perhaps that gets you in touch with people in need of specific expertise?
I honestly didn't use venues of this kind. I was fortunate to have some folks contact me. There are publishers who look for authors for book chapters. I found these to be too much work for too little compensation. I don't know if ifreelance.com or similar sites (search "freelance tech writer") are reliable but you have some time to research perhaps?