Customer Service and Membership Renewals as a Niche?

Ok, so customer service and membership renewals is my background. My understanding when deciding on a niche is to 1.)Choose what will sell!! 2.) Start with what you know & 3.)Choose one thing to focus on.
The place that I used to work closed their doors suddenly a few months ago. I would think that most of the clients that we dealt with would of course now need to find someone else. I would love to get in touch with them, but I'm not entirely sure how to go about it as I am a ONE person shop rather than a large call center and I would prefer to focus primarily on chat and email support rather than phone calls. And I'm not looking to do sales. I keep coming back around to 'how do I niche customer service without simply applying for job postings'?