I had to euthanize a tame rat and used this method. It was very peaceful. But ether is not easy to find.
An idea that soothes me about death is that by assisting our animal to the other side we are helping her be reborn into the next existence. It's like being a midwife in a way.
Ether is starting fluid. Can be found at any auto parts store. Even Walmart.
I believe there are different concentrations, I would just go with the highest concentration, and make sure there are no additives in it, just ether.