If you go to the your system Drive in My Computer (XP) or Computer (Windows 7) and then to Programme Files (X86 on 64 bit Windows 7), you then go to the Replay Music Folder, in there you will find the silence MP3. You can copy and paste it to the most convenient place on your PC/Drive.
To insert inbetween tracks on spotify just hold down your left button on your mouse and drag from the place you saved it on your PC to the Spotify window and inbetween the tracks you are listening to.
It will automatically place a Silence file of 5 seconds inbetween the tracks.
Also ensure that you have Gapless recording set in Spotify, go to Edit at the top and select Preferences in Spotify.
Hope this helps.
Steve :-)