I recommend looking at your own visitor statistics through free tools such as Google Analytics where you can easily dissect your hits by browser, OS, and browser version, as well as see the trends in how your visitors are changing. Even if you find an overall external source with "reliable" or "accurate" numbers your own site is likely attracting a subset of visitors that are not perfectly representative of overall statistics, whether due to geographic location, content, age or gender of most typical visitor, etc, which will all have an impact on what kind of devices are used to access your website.
Thus I recommend using the number of visitors to your site to come up with a personal threshold based on either numbers or percentage of hits from various browser versions. You might decide that if your visits from IE6 drop below 2% then you don't need to support it anymore. On the other hand, if you receive a million hits a day you might decide that 2% is still worth supporting, but you might go with a raw number and say any browser that sends less than 100 hits a day isn't worth supporting. The key is to come up with a reasonable metric based on your personal statistics and how many browsers you have the resources to test and develop for so you can find that proper balance between supporting everything under the sun and losing customers.