I run a productized service - we have over 20 clients and they all love it. Most of them were found either by posting about it on Facebook groups or referrals - that strategy seems hard to scale.
Looks like you are off to a great start. Curious to where you are today based on this question being asked ahile ago. I would actually look inward before worrying about how to scale the marketing and sales side of things.
You have 20+ customers already which is amazing.
Assuming you haven't figured out a great sales and marketing strategy yet I would look at the following.
1. How can you ensure you keep these 20 customers
2. How can you ensure your productized service has a recurrring aspect?
3. What can you do to increase the customer value and buy more?
4. How can you get your customers currently to buy more frequently?
5. How can you increase referrals from your current customer base?
