There really isn't anything as kind and gentle as what the vet can give her. However call your vet and ask about the possibility of a home visit for a euthanasia or look in the phone book for a mobile vet that would come and do this.
I'm sure you don't want to be cruel to your cat and you don't want to violate cruelty laws (which you might be by trying to kill the cat at home) so the best bet is the vet.
If all else fails your vet may be able to give you a strong tranquilizer so the cat will sleep on the way into the office.
Hope this helps you!
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