Okay, in english and in detail

The Umweltplakette cannot be bought, it will be issued for cars which fulfill the requirements.

Do you use your own car? Or will you rent a car? You can expect rented cars to fulfill the requirements and have a Umweltplakette. If you use your own car, I have to admit, I am not aware of the laws in Frankfurt. Here in Munich the Umweltplakette is not required for cars with a foreign license plate.