They’re two of the poisonous plant species with toxins known to stop the heart. Cerberin however is more powerful, but it’s not pleasant. You need to take it with something to minimize, if not eliminate the GI symptoms, which that would be scopolamine. If course, you need that for both, but when it comes to cardiac glycosides, those symptoms are most likely vagal.
For yews, you could probably get by with just making a smoothie, but for best results, it’s best to consume it as an alcoholic extract. Which with that, you need to get rid of the volatile oils and ephedrine; they impede the taxine. All compounds are soluble in alcohol, but where it differs is that taxine doesn’t dissolve in water, but ephedrine does, especially when it’s at a simmering temperature, oils are water insoluble, but they are volatile and may be removed with the water simmering. That said, I’d say petroleum either/heptane would also be efficient. Scopolamine is also the only antiemetic to work with the poison. Alternatively, you could use datura inoxia seeds as they contain scopolamine. 10-20 seeds is optimal.
One more thing before I leave you with those links, to ensure success, you need to go overkill. While they say for yews is 50-100 grams, do at least 2 grams/kg. And make sure to take at least 2-3 seeds for cerbeea odollam despite it saying the lethal dose is one seed.
Yew:
https://en.m.wikipedia.org/wiki/Taxus_baccata
Cerbera Odollam:
https://en.m.wikipedia.org/wiki/Cerbera_odollam
As for where they’re found; taxine can be found in graveyards, parks, the wild, or online if no shrub is available.
Cerbera odollam is on eBay.