Good question. I'm actually just helping a startup with this.
Suggestions (some obvious, some less):
1. Forums (in your relevant service).
2. FB groups.
3. Partnerships with HR people in that field.
4. Post Job opening.
6. Contact them directly, and give them an incentive to post/share that they've joined your platform (which will bring some of their friends).
There's a lot more - depends on the field.
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I consider this my most important role at my startup, which is a self-service, two-sided marketplace modeled after Match.com for facilitating product donations between companies and charities. From my B2B experience I would suggest:
• Professional associations: Find out where they get information to understand their priorities and challenges better
• Industry events: Meet them in casual, social settings
• Agencies and fellow service providers: Partnering with others that serve the same community, especially if your platform makes their jobs easier
• Testimonials: If you wow one or more well-known or influential clients in a specific industry, others who trust their advice and judgment will follow.
Lastly, I'd suggest networking, being generous with sharing your knowledge, and becoming a trusted expert in your field. You want to be the one someone thinks of when they, or someone they know, is seeking a solution to the problem you solve.
I would suggest you go through the list of B2B marketplaces and contact the suppliers directly:
Alibaba-The world's leading platform for global wholesale trade, Alibaba is the largest B2B online marketplace in the world.
Beldara- Trusted B2B online Marketplace for Buyer & Sellers is an online global B2B marketplace website linking worldwide suppliers to global buyers at the platform.
In addtiona to reddit, you can try following:
Post Job opening for suppliers.