What is the ruling on feeling uneasy and doubting the purity of wooden or sponge seats in waiting rooms in a non-Muslim country, knowing that the people of that country do not purify themselves after relieving themselves and their clothes may be impure?

These whisperings are baseless. The default state of seats is purity, and you should not rule something impure unless you have absolute certainty. Your assumption that people are not purifying themselves is incorrect; the reality is otherwise. What you are feeling are merely whisperings from Satan.