I wonder how many receipts could be acquired from the ground or garbage cans outside the store? It would be a biased sample, possibly, but it is hard data and free of cost.
Tastes vary (as the mix of comments here will attest), but having sampled a variety of development and analysis tools, I have settled on MATLAB as my tool of choice. Part of my reasoning can be found in the Nov-08-2006 posting to my Weblog, "Why MATLAB for Data Mining?":