The best way to startup will be to focus on networking effectively with the stakeholders/decision makers in the niche market. Build a relationship with them and demonstrate you are able to offer quality service.
Second is to do a survey on other recruiting firms offering similar services, gain clarity on their recruitment fees/rates and offer something lower than the market to gain a competitive edge.
Also ensure to have reviews of past client engagements posted on your website/social media handles that your niche market can see in order to establish reputation and trust.