Delta waves meditation is a difficult thing to achieve. Delta waves is so far from your waking beta state and to get there you must step into alpha and theta before reaching the delta state. Once in the delta state, it will be difficult to maintain a little bit of conscious awareness since the delta waves are produced when you are in deep sleep.
However, entraining to the delta frequency can still fire some delta waves and hence reap it's benefits. It's good for insomnia treatment, pain relief and generally for energy rejuvenation.
The following is a sample of isochronic tones delta waves which can be used for meditation. The tone has a regular up and down pattern to aid you in your breathing.
Other samples of delta waves isochronic tones and binaural beats can be found at the brainwave entrainment blog.