Of the top of my head, you can start out with a Google My Business profile so that people searching for related services can identify you when they live close by. This is totally free and setup takes 15 mins. Google does need a physical address for verification purpose however.
Other options include getting involved in social media groups that discuss about the specific topics, and offering tips and advice; just like I am doing right now. For an accountant, a quick search for “tax help” or “book keeping” in Facebook will show many groups.
Last and not least, spread the words about your service with your friend circles. I got my first ever marketing client through a friend’s referral. If they don’t know now, it could come up in the future and they’ll remember you.
All the best,