I don't know about "peaceful" necessarily but there are quite a few natural options out there that (I'd think) would be nearly guaranteed if over-consumed while isolated.
Some wild plants are a threat to hikers and amateur foragers, while other common flora are a hidden danger to kids, pets and farm animals.
www.mnn.com
View attachment 19852
None will be peaceful. Rosary pea sounded nice until I saw, "Symptoms of poisoning include nausea, vomiting, convulsions, liver failure, and death, usually after several days." And good luck on finding castor beans, especially due to ricin.
"In the US, ricin appears on the select agents list of the Department of Health and Human Services, and scientists must register with HHS to use ricin in their research. However, investigators possessing less than 1000 mg (1g) are exempt from regulation.
Ricin is classified as an extremely hazardous substance in the United States as defined in Section 302 of the US Emergency Planning and Community Right-to-Know Act (42 U.S.C. 11002), and is subject to strict reporting requirements by facilities that produce, store, or use it in significant quantities."
The death process takes several days for ricin, depending upon the dosage, but at least one advantage out of it is that it's almost impossible to detect in an autopsy.