You may be able to get this done at any emergency room "la guardia". If you have insurance (swiss medical, italian hospital, german hospital, etc) you can just get an appointment to have it done.
Also, I'm a personal trainer, and I run group yoga and pilates classes every week in my own studio.
Non-gyms won't ask for the test.
Go to the German Hospital, Hospital Aleman, on Avenida Pueyrredon and Juncal. Just tell them that though you are a member you are willing to pay for the exam to get this certificate.You will have to pass the test, run for 5 minutes , get tested for your cardio, and take the certificate with you.