I try to avoid this poison because I don't want to land on some kind of FBI terrorist list. With a little patience you could wait until summer, stear clear of any misunderstandings, and shoot for something like Poisonous Hemlock. Also, depending on your location, there are many places where Yew (taxus baccata) is available year round. Yew is available in virtually most of the USA and UK and I believe it's abundantly available in most of Europe.
Plus what @Walilamdzi said. The more i've reflected on poisoning by plants and the more I've researched the more I've heard horror stories and people who have failed to kill themselves. There's actually a case of a woman in Utah who failed to kill herself after consuming several castor beans. Sounds painful.