There are ample of sources, you can use for free marketing. The best one is having online presence. Facebook, Instagram, and even WhatsApp comes as a tool. Where the service industry may find it a little difficult to market over there platforms, these are surely helpful when it’s product based.
You can also consider about making videos on YouTube or IGTV.
While using these tools, you need to care for the quality of content being published on public domains.