While the terms data and statistics are often used interchangeably, they are different.
There is no single source / place where you can find government data and statistics / public opinion information online.
One strategy you can use when searching for data and statistics related to a topic online is to add the words data or statistics or poll to your search terms.
You can also restrict results to government statistics and data by adding site:gov to your search.
While including statistics can provide credibility to your argument, including faulty statistics can harm your argument. You need to evaluate the source of your statistical information. The following questions will help you to evaluate the reliability of statistical information.
Purdue OWL's Writing with Statistics guide is a great resource for helping you work with statistics in your writing.