There are only a few official maps available for reference in your mapsrc folder (under sourcesdk_content > tf (where your defualt vmf save location should be)). To see more you can either download the decompiled versions from the site at the download section (though i hear that one or two have errors) or decompile them yourself with custom tools.
Vmex (
here) will decompile bsp's, which you will need to extract using nem's gcfscape (
here).
gcfscape will be able to "read" the encrypted game cache files (gcf's) and you can select at will for extraction the bsp files (along with other official materials) in order to decompile them.
Have fun.