Am using Korg AW-2 clip on tuner. Works well on my 5-string bass (low-B) while clipped to the head. Am also using it on my electric double-bass while clipped to the bridge.
http://www.korg.com/product.aspx?pd=552
Bought from Citymusic like the 2 previous posts.
Important to leave your fretting hand off the neck while waiting for the tuner to register the note before turning the machine head, just like what was mentioned earlier. Using the piezo crystal requires the bass to resonate well without damping.