It all depends on the amount of users you need, how targeted they need to be, where online they are active and the way you approach them. One of the problems is scalability... You may have identified some online communities in which your potential users are active, but trying to reach out to each and every one of them is a time-killer and not scalable if you do that manually!

What worked for me in the past, and is also working for the Startups I advice to today - is the development of automated tools that will reach out to your target audience. The market/industry is irrelevant, as long as you can identify where online your target audience is active, and then build tools around these websites/platforms.

In my current Startup I've used this technique to acquire 3,000 beta users for a business networking app. It didn't take long, as it was VERY targeted (converted very high! almost 10% of the people we reached out to ended up installing the app!)
This was also the case in a previous Startup - different industry, but same approach. I developed these tools for the purpose of creating a beta community. It was so powerful that we kept using it long after the beta ended. 5 years later we reached over 100M users! (around the first million came with no marketing $$ spent)

With that said, it may take some time to build these type of tools, so it all depends on the amount of users you're trying to get, as well as the cost of alternative solutions. The good news is that you can keep on using these tools for growth and user acquisition on later stages too.