I used to work on a freelance basis in a similar domain, but was hired by a client for a full-time position. While I do not have much time or interest right now to add freelance work to the schedule, I don't want to lose that network and do accept the occasional project if the fit is right and it's a good investment in the relationship. My approach is fairly casual, but once or twice a year I scan through my list of contacts for people who have the potential to hire for the work you could take on. Then I keep in touch with a short, but specific, note, something like, "Hey, hope the new year is treating you well, things are going great here and I'm enjoying the job, etc., but I have a little time to take on some freelance work in X,Y, or Z. Let me know if there's anything in your pipeline that we might work on together."
I have always found it easier to get freelance work through former contacts than through cold channels where I'm responding to an open-ended request.
And if no one in your immediate network is in need, you can have a similar conversation but ask for introductions to people in their network who hire freelancers.