Kindly advise the ruling of eating non halal food by mistake. how can one cleanse oneself? and in a situation where you dont know a place, can you consume non halal food since it's a new place and you dont know the halal places in that particular town or city..

If it is by mistake ask for forgiveness sincerely and be careful next time.
As for if you don’t know the halal places then you must avoid all the meat and chicken.