Interestingly, the problem you face is probably the #1 problem that many businesses face - how to grow a customer base through marketing.

For your specific business, the three channels that I would diligently try and TEST OUT are;

1. Word of mouth ( you're doing this)
2. Content Marketing
3. Traditional Banner, Facebook, LinkedIn, etc.

With my company, C&M Group, we are able to reach clientele outside our immediate network in the US just by using these 3 methods. The best part - the top 2 that I mentioned are absolutely free and just require immense time (which is why people don't do it).

Let me know if you would like to hop on a call and discuss this further.