In my experience, online ads don't work that well for small professional services businesses. As always, YMMV.

The best leads are always referrals. After finishing a job, make sure your client knows that you're looking for more work. If you did a good job for them they will send leads your way. This takes time to build momentum but it attracts the right kinds of customers, and your conversion rate will be way higher than for cold leads.

Good luck!