What data is used to calculate the CPI (Consumer Price Index) and if it is open, where can I find it?
The CPI is based on surveys conducted by BLS (Bureau of Labor Statistics). Those surveys collect price information which is reviewed by commodity experts. Those experts review (and possible adjust) the data based on their knowledge of the particular commodity. That adjusted data is then averaged to calculate the CPI. You can definitely get the values that are averaged in the final calculation. I'm less clear on how to find the actual survey responses.
Everything dvogel wrote is correct. Note that on the ftp page there are several datasets (including "ap" and "cu") which are labelled "consumer price index."
The Bureau of Labor Statistics (BLS) samples posted prices at various US businesses on a monthly basis. This sampling includes roughly 85 thousand observations per month. The BLS then creates and publishes price indices broken down by the type of good and the geographic location where it is sold. The original data are not made available to the public, but researchers can apply for restricted on-site access. See (http://ideas.repec.org/a/ucp/jpolec/v112y2004i5p947-985.html). The underlying dataset is referred to as the "CPI-RDB" (Research Database).
The BLS does not publish documentation for the underlying dataset-- their website does not mention the CPI-RDB or the option of applying for access. The best documentation I am aware of was written by an intern circa 2001-2005 named Teague Ruder, now at RAND (http://www.teagueruder.com/portfolio/projects/RDB.pdf).
1) Be careful when using data from the BLS ftp page. Many of the datasets contain typos or missing tab delimiters. As important, the geographic codes are not consistent between datasets.
2) The Billion Prices Project at MIT is an attempt to recreate the CPI data by scraping online retail. Some of their data may be available.