I don't think you could get one like that. To get a certificate, you need to take a test, for example Cambridge's PET, KET, CPE etc, or the TOEFL. They are not cheap (the TOEFL is about 160 USD), but they are internationally recognised.

You might be able to get a fake one somewhere but it will cost you a lot but when you get found out, it will not be good for you.