Once your ideal client profile is established, how do you find the company's decision maker and how to reach out to that person?

There are excellent answers here already, my comment is to add to those: network amongst your clients, before your peers and competitors.
Join the associations your clients join, attend the events your clients attend, learn through the same channels your clients use. This is how you will know who to target and how.

Network and ask the correct questions. If you find them interested in what you have to offer, simply asking a question like, "what would it take for us to move forward?" can be very powerful.

I'm specialist in Sales excellence and had been worked for Monitor Deloitte for 7 years on this topic. Depending on your business, I'd suggest you not to define your client profile based on volume, geography size or segment. I'd rather to defening it based on purchase behavior(But it depends on your business). You are probably having hard time to identify the decision maker because of an inefficient segmentation criteria(ideal client) Once you review your segmentation, your will look for company contacts on you CRM, if you don't have one yet. do implement one. If there is no contact on your CRM, ask people of your company to find people from the prospect company (supposing you already have, at least, the company name) on linkedin, if not, ask you contacts, that you know have relationship with anyone on that company on linked in to introduce you to them. I hope it helps. If still you do not find anyone, let me know about your business so that I can go a little further on this topic and help you out.

The answer is a question. What departments in your prospect companies does your product or service touch? Are you providing a high-performance sales product or service? Identify the head of sales, head of development or customer relationships. Often company leadership can be found on the website, Crunchbase or Google. Always establish relationships by phone and schedule an appointment. If your company is a startup and your prospect is an established company, don't treat the director or vice president like a peer. Emphasize the value that your product or service represents to each company succinctly and specifically to gain the appointment and close the sale. Follow up your scheduled appointment with a snail mail thank letter along with a two-page prospectus outlining what your company will accomplish for their company. Cheers! To finesse pitching your product's value proposition simply schedule a call.

You ask two questions:

1. How to find the "decision maker"
You make a potentially dangerous assumption here. Understand that there may be many "decision makers". Most of them can say no and nix the deal, but (usually) only one of them can say "yes" and the sale will happen.

Is your sale simple or complex? If simple, i.e. there is only one person involved in the buying process, then all the answers here are correct. If your sale is complex, i.e. there are many people involved in the buying process then you have a very tricky problem as each of them may have a different perspective on the business problem, the way to address it and how important it is.

2. Ideal Customer profile
What you describe, "companies based on revenue, geography, etc" is the target market, it is not the ICP. The ICP talks more about the issues and problems that you solve. So, an ICP within a target market would include factors like:

- Companies who are growing rapidly and who struggle with communications among remote employees
- Companies who require up-to-date information to employees while out of the office
- Companies who face tremendous pressure to grow despite a down market

About me:
I deal only in the world of complex sales. If you want to talk about selling this this world, then contact me. If you are in the simple world (not that that is a bad thing) then one of the other experts will be able to help you much more.