Where in Stack Overflow can I ask, which software is best suitable for specific purpose? For example, a good software which is free for fashion related. The application should be able to show a preview how a person looks in different hair styles and eyeglass frames etc.
Nowhere. :-) None of the sites in the SE network are designed to offer software or product recommendations. Google and Bing both specialize in searching for things and returning links to the results, and there are many software review sites that will help you decide between them.
Note: Now there is in fact such a site at [softwarerecs.se], which has been created in the time since this answer was originally written.
At present there is no Stack Exchange site where you can ask such questions. But there is a proposal in Area 51 for such site where you can ask for software recommendation.
Proposed Q&A site for people seeking advice and help finding software to get tasks done.
You can follow the proposal and help it to launch in near future.
Since today we have a new SE site called Software Recommendations.
Note that this is not a site to ask about the “best fashion software” or a list of “fashion-related software”! SR.SE has the same expectations of answerable, useful questions as other Stack Exchange sites. Questions must lay out a goal or task to accomplish, and specify some objective requirements. Please read the question quality guidelines before asking there.
According to the Help section, that question isn't allowed on SE. Please take a look at the above link to see what questions are acceptable here.