Ok the maps are in huge files that are packed tightly within your computer!
Mine are in:
C:Program FilesValveSteamSteamApps
You will see this:
http://img8.echo.cx/img8/5411/12yi1.jpgThese files contain what you are looking for, you will need gfcscape to open these type of files!
The maps for HL2 are in here:
http://img269.echo.cx/img269/2477/18xh.jpgUse gfcscape to open this one and tell it to extract the map you want to the desktop e.g. if you want a Ravenholm map you must choose which one and right click, extract to desktop and presto.
Now to decompile .bsp which is wrong and you must NEVER steal other peoples work and the point of decompiling properly is to see how an effect is done.
Ok so if you agree with this go here:
http://www.geocities.com/cofrdrbob/Download the latest decompiler and install etc.
Follow the instructions to decompile it because it is hard to explain but rather easier if you try it yourself!
Once you have a decompiled .vmf file open Hammer using the sdk and presto! Look at the bits that is used to make dog play ball and copy over
Note that you can't steal any parts of the map and copy it over to yours because it will damage your map and make it not work!
Only models and entities can be copied safely and thats all you need!
Good luck I guess...