There's cerbera odollam seeds. The cerberin in them is undetectable in autopsies and thus can be mistaken as a heart attack. However, if you don't add the other ingredients, Like scopolamine from Datura Inoxia/Brugmansia seeds (minimally), incarvillea sinensis, California poppy, and akuamma seeds. It will be unpleasant.
There is also the suicidal adrenaline technique, not that I recommend it as it's just theoretical and is still somewhat in development for perfection, and you'll likely end up fucking yourself up completely of you don't go all the way. However, should it work, it will be natural as it only involves your body and you'll either giver yourself a heart attack or a subarachnoid hemorrhage. I still need to post the updated instructions.
Other than that, if no one suspects you of being suicidal, you could also screw around with loperamide and pgp inhibitors. Or try overdosing on otc or prescription drugs thar addicts use.