You probably know this already, but for the benefit of others: While it depends on the subfield, a lot of computer science and mathematics papers are freely available.
1. Searching for the title often gives you a PDF in the first few results already. Authors usually have the right to upload a version on their personal website or arXiv.
2. There is Google Scholar [1], which has links to PDFs on the right of the result (if it knows one). It is better at this than regular Google search, in my experience.
3. Manually searching on the author's website (or former website, if they moved) sometimes proves successful, although this is relatively more effort.
4. As mentioned, writing an e-mail to the authors works as well. (If you buy a paper, the authors get nothing, so there is no incentive not to share it.)