Unfortunately such pills aren't available OTC. You may try your luck in the darknet but it comes with risks imo.
Edit: Maybe this could be of interest for you.
Translation from the German book "Selbstbestimmtes Sterben. Sanfter Tod bei klarem Geist" Victor Niculescu Hypothermia & Diphenhydramine(chloride) "The lethal dose of Diphenhydramine(chloride) is 50mg per kg body weight. also required: Zopiclon 20 tablets Bromazepam or Triazolam 30-50...
sanctioned-suicide.net