I admire your ambition! To answer your question directly, I'm not familiar with any site. Perhaps a message board you're familiar with? Has your instructor addressed the issue of bias? It may be enough that you recognize the issue of bias in your sample and speak to how this will affect your survey. Most properly randomized nationally representative surveys have an N of somewhere between 1800 and 2300. [The GSS probably has a few questions regarding global warming][1].
However, this will be inaccessible if you're not familiar with SPSS or Stata. I know others on here can help you better than I in regards to weighting responses to correct for bias. 

  [1]: http://www.norc.uchicago.edu/GSS+Website/