To find the version that fits your needs, please read the versions page listing.
NOTE: To work effectively with Sandy you should have access to the latest source and fixes. Therefore, you should obtain and use an SVN client to download the latest source tree for whatever version of Sandy you intend to use.
Use an SVN client, not a web browser, to download the desired source tree from one of the path's below (or take the entire tree and then point your ActionScript classPath to the desired subfolder). I recommend TortoiseSVN for Windows (see list at previous link for other choices).
Here are some links of interest (use these links to download–i.e., export–the latest source tree using an SVN client. Visiting them in a web browser won't help much)
See also flash_version_support for more details.