The first thing really to decide is what sort of path you want to take. Are you going to 1) be self employed working on projects, with multiple clients at once, or 2) be looking to work on a single 9-5 contract where you're basically treated as an employee?
Option 1 (best IMO):
Your first job really is to find contacts and get known in your area. Your second job is to sell yourself to anyone you meet.
In your position, I'd start by attending all the relevant user groups, networking events, coffee mornings (might be hard if you're still working 9-5), whatever in your area. Try and find a good co-working space in your area too and see about getting involved in any events they have going on, if it's cheap enough, it might even be worth joining a co-working space. Then often have internal mailing lists where leads are shared or work is subcontracted.
Option 2:
Just treat it like finding any normal job, look on job boards, contract recruiters etc... For your first couple of roles, these will be your sources. Be prepared to take a lower rate than you're maybe expecting until you get some experience and contacts in your area.