How do I find customers online for the negotiation services business I am about to launch?

You're going to want a comprehensive marketing strategy that determines where your likely customers spend their time online and you'll want to focus on one to two channels that you will work to their maximum benefit until you feel they are saturated. You probably also want to think about enabling technologies that will allow you to scale your inputs so you don't have to wear yourself out services clients while your business development suffers. There are a lot of "it depends" in your question. It's not enough to "find" if you don't have an engagement strategy of what you will do when you find them. I'd be happy to talk through this with you and point you in the right direction. All of my work is related to B2B sales of this nature.

Show up at Meetup networking events.

My guess is if you press flesh with humans, you'll make a truckload more evergreen money, than using social media.

Try this experiment + see how it effects your bottom line.

Working Meetup events is similar to Dan Kennedy's suggestion the best way to fish is with a barrel of fish + a shotgun or dynamite.

How do you get your first 10 customers?

First, forget marketing. It’s great but, a little slower to bear fruit. Plus you do not know your customer profile yet. Today, is about making your first sale.

Second, forget hiring a sales person… yet. You need to figure out how your solution is sold before you know what type of sales person you need.

Get out of the spreadsheet and into your client’s office.

I will skip over pre selling as it sounds like you are past that.

Sell now
If your product is ready to start selling now then make a list of 10 ideal targets from your own network. Choose people you have worked with in the past that are in your target network. The reason being is that these are warm prospects where you have already built up trust and rapport. Be hyper focused on the right market and customer type. If someone is a maybe fit, mark them as not a fit for now and select someone else. Don’t make the mistake of trying to sell to everybody. It doesn’t work!

Prepare your conversation.
Prepare a very basic script of what to discuss with the prospect. Focus on the problems that your solution can help with. Ideally these should be common problems that the market experiences. Create 5-6 questions that you could ask. Rehearse this a bit until you feel like the conversation and questions sound natural.

“Higher-performing sales representatives ask prospects more questions–often more than twice as many. Their questions are more focused on implications than on data. Put another way, they ask questions about what something means rather than just what it is” according to author Tom Searcy.

The questions should help validate the problem the customer is having, validate if your solution is the right fit and determine if they are willing to pay for it. The answers determine how you describe your solution to them using the prospect’s own words as much as possible.

Some examples of questions might be:
Would you mind telling me about your current situation?
What would you say is one of the top three priorities you are grappling with right now in your company?
What’s not working?
If (Your solution) addressed (repeat issues raised back to them), would you be willing to pay for that?
Would you make a purchase order for it now?
Given what we have discussed here today, what would it take for you to become a customer?
The Rain Group have a list of 21 powerful, open-ended sales questions around rapport, aspirations and afflictions, impact and envisioning a new reality.

Reach out and meet with them.
Outreach via phone, email or LinkedIn. Be persistent until you talk to everyone and ask if you can meet them. Those that say no, ask if they can recommend someone else instead.
You will need lots of persistence at this stage. Even prospecting within your warm market requires lots of touches so do not get discouraged by lack of replies.
The Bridge Group, researched 355 of the leading SaaS companies and found that sales people make on average six to eight attempts before throwing in the towel. However High performing sales people make up to 12 attempts.

Sales development reps number of contacts per lead

Meet with the 10 prospects and be upfront with them. Explain that you have a new solution that you are looking to validate in the market. But take the pressure out of the conversation and explain that you are looking for their expert feedback on your idea and solution. A sale would be the ultimate validation but either result is a successful meeting for you. This takes the pressure off the prospect and allows them to relax more and hear about your solution.
Your mindset is key. You are not going in to sell them. You are going in to find out their problems and see if you can help them solve it.
Finish every conversation with the question “What would it take for you to become a customer?” This determines your next step.

Did you get a sale?
Yes? Well done.
No? If you can’t sell your product or service to someone who already knows, likes, and trusts you then carefully consider if you are talking to the right type of people and there really is a market for your product. If you are really really sure, step it up a notch.

You need to widen your net. If you are selling business to business start by identifying 100 companies you can sell to in your local area (drivable distance). Start calling them and if anyone shows interest then go and meet with them. At this stage of the business you really need face to face meetings to hear their issues, see their reactions when you describe your product and to understand why they are saying yes or no.

Your first sale, it does not matter too much about making a profit on the sale. Travelling and meeting face to face will probably not scale for your business. The points you are trying to prove is that 1. someone will pay for your product (and hopefully pay your asking price). 2. you are targeting the right market. 3. Get an example customer to show your next prospect which will make the sale easier.

I go into this in more detail here if you are interested: https://www.onepagecrm.com/blog/how-does-a-small-company-make-its-first-b2b-sale