Usually behind the successfull business stands the team of experts from several domains (software, marketing, finance...). In the past I've had some ideas which I still believe could be a story of success if would be realized at that time but working as one man band is difficult and to find reliable people is neither easy.
Does anyone know any address online where people establish contacts with purpose to launch a business together or any event that is worth to attend in order to expand professional network?
In any city around the world there will always be entrepreneur / startup groups. Google is the best place to search for these. They will usually have a meeting once a month or more frequently.
The other option is to try networking with your local business school / college or anywhere that they offer business training or MBAs. If they have a public web site or on campus they may have a notice board, you should be able to find notices regarding meet-ups or other social events. If you aren't having any success try and find out some class times and catch up with some of the attendees after the class or even try having a word to the lecturer.
It's all about interacting with community, so try and find where your local community is then start interacting. You could try posting another question (or searching) for something along the lines of "Are there any startup groups in (your local area)?".
Sometimes, a prospective partner approaches me with an idea based on my reputation in the Drupal community (I'm a web developer). I've even blogged about what makes a speculative project worthwhile from a developer's perspective. I've found partners among fellow volunteers on open source projects, long-term clients, and am even working on one project with a family member. This time around, I'm trying out an event called Startup Weekend (google it, I'm too new to post more links), to see if anything grabs my interest.
In my experience, finding the right partner(s) is a matter of meeting and working with a lot of different people. The more you are exposed to, the better your chances of meeting someone whose talents and personality compliment your own, and who is interested in the same opportunities you are.
I use LinkedIn to meet other entrepreneurs in my domain. I network with these people by interacting on LinkedIn groups. I had great success through this approach. I am business consultant and I regard myself weak in coding software. I pitch my idea to these group of people whenever I think a idea can be a big business.
I am regular speaker at NGOs which concentrates on promoting education. I mentor students on entrepreneurship and management issues and this helped me to generate entrepreneurs which can be my next partner in business.
I also approach persons through mutual friends and discuss business over a cup of tea.