Is there a site to post my survey to so I can get a sample representative of the population? This is only for my high school senior project, so it doesn't have to be perfect. I'm doing a project on global warming, and I want to survey people for their opinions. I know that if I use a convenience sample of my classmates, I'll have a lot of bias. I was wondering if there was a site on the Internet where I could post my survey for random people to answer, so that I could get as close as possible to an SRS. If not, I'd be interested in some other suggestions for ways to carry out my survey. 
 A: Your best bet is mechanical turk, https://www.mturk.com/mturk/welcome. It will cost you some money, but not much. If your questions are short and can be done in ~ 1 min you can easily charge 10 cents or so per answer, so if you want 50 responses it will cost only $5. You can get the data formatted in .xml, which you can import into R or even (shudder) Excel. The people you sample will be much more varied than you'd get in a high school class. And you can ask for demographic info (age, gender, nationality, etc), as questions in your survey. Also making the survey is easy, just adjust amazon's templates. 
See http://papers.ssrn.com/sol3/papers.cfm?abstract_id=1601785 for more issues with the biases you mention, the WEIRD study. Also see why people at mech turk participate: http://behind-the-enemy-lines.blogspot.com/2008/09/why-people-participate-on-mechanical.html. 
A: 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.
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. 
A: For a high school project it will probably be difficult to gain a representative sample with your limited time and budget. I'd be somewhat skeptical an online vendor can achieve a random sample of anything. If I had to do it I think I would use the phone book and either call or go to the addresses to distribute the survey in your community. Although this isn't perfect (likley under-represents younger people, minorities, and people of lower socio-economic status) it makes the project more feasible in your situation (as opposed to conducting stratified sampling based on geographic regions). Both of these solutions are local samples, and would not get you a national sample. An agency I work for samples the local community based on parcel addresses and apartment listings obtained from the postal office (these sources of information are not always public though, and would require more front end work than opening up a phone book).
idclark is right and the General Social Survey has several variables addressing public opinion on global warming. You could either analyze that data directly (as it is open to the public) or if you are required to construct your own survey you could mimic the GSS's questions and then see how close your sample appears to be compared to the GSS. At the moment I am having a much easier time searching the GSS variable lists from the ICPSR archive than I am from the actual GSS website. You can actually conduct basic data analysis online at ICPSR of the GSS data (you can basically do frequencies and cross-tabulations). If all you need are frequencies this circumvents the need to use stat software such as SPSS or Stata. 
Getting a representative sample is difficult not only because true populations which to construct your sample are sometimes hard to come by, but just as importantly not everybody you do sample responds to the survey. I wouldn't be fixated on a national sample either, I'm sure you can construct just as interesting questions to answer by estimating public opinion of the local community, school mates, teachers, etc. What I find interesting is in comparison (like say comparing your school teachers opinions to those of the students). You could also conduct an experiment via surveys (like say manipulate how you present arguments for or against global warming and see how people differ in their opinions of global warming).
A: a sample representative of population cannot be obtained through internet as you will only get people interested in answering your survey online, which will give you a biased sample.
A: Marketing people are using self-selected samples in online-surveys all the time, so they probably have methods for cleaning their data. Sadly, I don't have a good pointer right now to look for their methods.
A: Just a thought, but it might make sense to implement an inexpensive Google Ad Words campaign where at least the participants would be coming directly from Google search and you can somewhat control some sort of stratification.  Of course, it is never possible to get a true sample.
Along these lines, there is some work being done nowadays to adjust for selection bias of Web Surveys via propensity scores. This is being done by some of the majors, including Harris.
http://www.amstat.org/sections/srms/proceedings/y2004/files/Jsm2004-000032.pdf
-Ralph Winters
