Consistent source of referrals is by referral. I keep in contact with my clients before, during and after closing. I send a monthly enewsletter and also a mailed letter with an item of value (I belong to the Buffini Network that provides). I make phone calls, text when I happen to think of them, send notes etc. I have also received referrals via my online presence and reviews. I have tried Zillow, Realtor.com, Homes.com etc in the past to pay for leads - but I felt like I was flushing money down the toilet. At that time, most of the leads did not translate into sales (often they already had an agent!). Have a presence on social media - business FaceBook and Instagram, so you stay top of mind to those who may have forgotten you sell real estate.
Honestly over the years I have tried numerous and not one has produced valid clients. However, that being said I have some friends who use zillow and have done well.