Have a look at some of the boxes for sale at ExtremeRestraints or the Stockroom. You need not get the most expensive until you have figured out which sensations you like best. You will be able to get replacement electrodes (sticky pads) at most medical supply houses a lot cheaper than adult sites. I have also done a lot of digging into the above the belt taboo, and my sense of it is that it is nonsense. A technical explanation is that these boxes have some voltage but the current that comes out is measured at most in milliamps and travels over the skin, not through it. This should not have any impact on your heart at all as long as you don't connect to a piercing or anything else that goes through the skin. Its no different than what a chiropractor would do, or for a more extreme example, what happens when Tased by a policeman. It doesn't kill you but from what I have seen it hurts like hell.

Most of all experiment. There are so many possibilities with electricity or e-stim, that you are sure to find some that are great for you.