You got some great suggestons in comments. You're looking for a good "phantom". It's going to depend very heavily on what signals and electrical properties of the body you're interested in. Assuming you're interested in some sort of electromagnetic properties that might interfere with your network, I'd look first at some sort of animal carcass. Another thing to consider would be a big porous sponge (like a carwash sponge) soaked in saline (0.9% as mentioned in comments). I've had students use that for electrical stimulation studies (electroporation, specifically). A Phantom for skin would be a bit tougher to implement, maybe thin leather, or maybe even a balloon, but neither would be real close in terms of resistive properties. I suspect for wave transmission, a balloon might be good enough, as the "bag of water" will dominate behavior.
Of coarse, there's all sorts of gelatins for the physical properties. I've seen ballistic gelatins and even the sorts of materials used for fishing lures used.
This paper talks about many more options for very accurate phantoms. http://uwcem.ece.wisc.edu/pdf/lazebnik_pmb05.pdf
Generally, though, if the circuit isn't dangerous, I'd just recommend using people. Yes, we breathe, and so will all your users. Try a spectrum of body types. If you notice problems, then you might choose to move to a phantom.
Things may change a bit if you're trying to measure biopotentials, and you need to generate the potential and watch it propagate through the body. If so, let me know and I'll try to edit.
As a last point, sometimes a bit of finite element modeling can go a long way toward validating a biological phantom.
