Great question
IT depends on how fast you want/need to get them. You mentioned "organic" and "cost effective" - these are two very different methods.
Organic - you can do a lot of content (articles and videos) and this will get you leads/customers - but you will only start seeing the effects after 4-5 months.
Another option is to partner with other complimentary organizations who already have traffic - your service would compliment theirs.
You can also focus on posts on the platform on which your relevant target audience exists - for example LinkedIn or Facebook...you could post in groups (some are huge).
As for cost effective - the obvious channels like Google or FB ads are a bit expensive and if you don't know how to do them right, you'd be wasting money before seeing results. So I would try something a little more out of the box - for example: specific forums/whatsapp groups etc, where your target audience is active.
It all comes down to who and where your target audience is.
Good luck
I am a mentor on Startups.com, an international lecturer on entrepreneurship and negotiations, a commercial startup lawyer and a leadership coach. I've successfully helped over 400 entrepreneurs, startups and businesses and I would be happy to help you.

You have to have faith and good customer service its hard to get a first customer but after you get the first person as your customer and you did give her a good services he or she will be the one who will bring other customer to you

Find a place (Upwork, craigslist, etc) where your customers are talking or posting that they need a service that you provide. Use IFTTT and create an alert when a new post is made. Use Zapier to move the data into google sheets and set up an email autoresponder with a message like "Hi. I have a ____ expert available in your area that has availability in the next week." Then just give them instructions on how to sign up. Almost no cost to connect with people who have a specific need for your service.

Hi! I'd take a look at the existing marketplace data / the business you have currently been able to book successfully and see what has been working. Example, are plumbers / handy type home projects most frequently booked, vs admin tasks, or deliveries? Then find businesses and local contractors in those areas, in your target markets and see if you can get them onto the marketplace. Referrals will be a major part of the success of this platform. I'd like to understand what your marketplaces advantages are, price, platform, ease of use, quality / trust to better understand your potential lanes of growth. Give me a call, lets chat!

In order to achieve the first 100 clients there are many different ways of fulfilling it. You cannot miss any of them. It requires a lot of patience, hard work, due diligence and consistency.
Leverage Social Media:
• Choose the right platforms: Identify where your target audience spends their time online (e.g., Facebook, Instagram, LinkedIn).
• Create engaging content: Share informative and relevant content that attracts your ideal customers.
• Run organic contests/giveaways: Generate excitement and grow your audience organically.
• Engage in conversations: Respond to comments and messages promptly to build relationships.
2. Content Marketing:
• Start a blog/website: Share valuable content related to your products/services to establish expertise and attract potential customers.
• Guest post on relevant blogs: Reach a wider audience by writing guest articles for blogs in your niche.
• Utilize video marketing: Create engaging and informative videos to showcase your offerings and connect with your audience.
3. Build Community Engagement:
• Join online communities: Participate in relevant online forums and groups where your target audience gathers.
• Organize online events: Host webinars, workshops, or Q&A sessions to connect with potential customers and provide value.
• Network strategically: Attend industry events and connect with potential customers and collaborators.
4. Referral Programs:
• Incentivize existing customers: Motivate your initial customers to recommend your business through rewards or discounts.
• Track referrals: Utilize a system to track and manage referrals effectively.
5. Utilize SEO (Search Engine Optimization):
• Optimize your website: Implement basic SEO strategies to improve your website's ranking in search results, leading to organic traffic.
• Target relevant keywords: Research and target keywords that your ideal customers are searching for.
Remember:
• Be patient and consistent: Building a customer base takes time and consistent effort.
• Track and analyze results: Monitor the effectiveness of your strategies and adjust them based on the data you gather.
• Focus on providing value: Build trust and attract customers by offering genuine value through your content and interactions.
By implementing these organic and cost-effective strategies, you can start building your customer base and pave the way for sustainable growth.