That's a tough one. The technical answer is that the .xsl files start at /_layouts/xsl/main.xsl and /_layouts/xsl/local.xsl, which call more files. However, customization of a list or library will be embedded within the aspx page you're using to view it, and the xsl is "by exception" for anything different than the default templates. SPD creates a whole lot of that exception code automatically. You can try to reverse engineer the main, local, and other xsl files, but that's a herculean task. If you have a specific list and a specific goal, you could post your WebPartPages:XsltListViewWebPart code here or in a new question and someone with SPD could make the changes for you.
One alternative, which you might prefer since it'll leave you in control, is to set up your page to be as close to your desired design as you can, then save the aspx file locally. Using a standalone (maybe even personal) computer, install SPD and (if it comes to it) a copy of SharePoint Foundation. Add your aspx file to that, use SPD to make the changes, and copy the file back. Be sure to keep an eye on the list and view guids, since those'll change between the two, but otherwise they should be the same.