It's absolutely possible to get customers / clients without doing any paid advertising.
As you suggested - this can be from word of mouth or from a variety of other sources.
Effective marketing plans often utilize a broad spectrum of both inbound and outbound strategies. And some business models lend themselves incredibly well to methods that involve little or no money. Other strategies / techniques / platforms require a bigger investment. (Notice I used the word "investment" not "spending")

In my opinion, the key is to create a business model that allows you to maximize your return on investment (ROI) for all of your marketing (and quite frankly just ALL) dollars.

For guidance on how to create such a business model - give me a call.