citations - How can I get the DOI of a paper from its title?

Is there a way to get the DOI (Document Object Identifier) of a research paper when its title is available?

I am preparing a reference database I could not get the URL of all the references. So, I tried to search online but could not get to anything. I will appreciate if anyone uses such tool/website or has any idea.

EDIT I am sorry but my problem is little bigger. I need to automatically (not manually) get them from the websites. Of course, I do not want to do it for more than 50 papers if that is legal/allowed.