It's the right approach, but you've failed at the penultimate hurdle.
You will most likely have to research such companies yourself to target them with an unsolicited letter.
Because this is such a specialised question it is really unlikely you would find someone on here just happening to know this.
However there is a really good chance such companies use to internet to promote their services - so search on the basis of identifying companies through you wanting their services - not because you are job-hunting. Then phone the company and ask reception the name of the MD. Then write to him/her.