The legal situation is a little trickier than it might seem. Even if you've got consent, could you actually prove that in a court of law? What about the fact that amputation is a sort of surgery, so effectively someone is practicing medicine without a license? You'll want to plan things out to minimize such risks, which means getting competent professionals involved, and not having them turn you in to the police during that planning stage, which means all the discussions have to be covered by either attourney/client privilege or doctor/patient privilege. Maybe start out by contacting one of those sketchy ambulance-chasing personal injury lawyers who advertise on TV, but don't spill your guts as to what the goal is until you've actually paid them so privilege applies.

Speaking of medical stuff, you'd want to have some significant preparations for aftercare. Some way to stop the bleeding, and some way to prevent the stump from becoming infected. Killing someone is pretty much always a crime, even if it was an accident during consensual activity.

Overall, be prepared to spend a shitload of money on this, even if you find someone eager to get eaten. Make a list of every unsexy way something could go wrong, and then come up with ways to prevent each complication, and ways to mitigate the damage if it occurs anyway, and implement them all. Do your research before committing.