There are lots of peaceful painless methods now days. We have lots of megathreads and I'm sure there is an index.
Suspension hanging, SN, fentanyl, pentobarbital, h2s, co, inert gasses are all quite well covered on here.
Also important to define what peaceful means to you. Some find pentobarbital (N) or SN as peaceful but to me whilst they are physically peaceful, I think they'd be emotional torture. Suspension hanging could be painful for 30 seconds but then out cold.. each has to make their own evaluation.