I know that the location for system apps is '/system/app' and the location for user apps is '/data/app'. But I can't find the location of apk for the ones that I moved to/installed on sdcard.
Apps installed on the SD-Card are stored within the directory ".android_secure".
If I remember correctly you won't find the APK files directly in that folder because the content is encrypted. This is for preventing direct access to the APK file of paid apps.
The encrypted part is mounted somewhere else into the file system.