I'm an Electrical Engineering grad student specializing in biomedical sensors/devices. While such a device is absolutely possible to design and fabricate, it would never be available on the market (at least in the US). The FDA would certainly consider this a medical device, even if you tired to market it as a "novelty item" to get around the regulations. Ignoring the fact that the device would do intentional harm to it's users (and therefor be outright rejected by the FDA), the cost associated with getting past the legal red tape is astronomical. There would have to be a huge market to overcome the barrier to entry. So really your only options are to make it yourself and prey you know what your doing. There are only a few ways of getting it right, and an endless number of ways to get it wrong... some tragically so.
Frankly, I could never understand the desire to be incontinent. It's like a less drastic version of body integrity identity disorder - where people want to amputate their own perfectly functional limbs. Being incontinent is not the worst thing imaginable, diaper really do make it bearable (though I really wish they became more socially accepted),