Great question
IT depends on how fast you want/need to get them. You mentioned "organic" and "cost effective" - these are two very different methods.
Organic - you can do a lot of content (articles and videos) and this will get you leads/customers - but you will only start seeing the effects after 4-5 months.
Another option is to partner with other complimentary organizations who already have traffic - your service would compliment theirs.
You can also focus on posts on the platform on which your relevant target audience exists - for example LinkedIn or Facebook...you could post in groups (some are huge).
As for cost effective - the obvious channels like Google or FB ads are a bit expensive and if you don't know how to do them right, you'd be wasting money before seeing results. So I would try something a little more out of the box - for example: specific forums/whatsapp groups etc, where your target audience is active.
It all comes down to who and where your target audience is.
Good luck
I am a mentor on Startups.com, an international lecturer on entrepreneurship and negotiations, a commercial startup lawyer and a leadership coach. I've successfully helped over 400 entrepreneurs, startups and businesses and I would be happy to help you.