I'm not really sure what you mean by "find the exam at 'Oracle University' " The exam information page is at the following URL, but that's not where you would schedule the test. The link to that page follows:
I logged in to Pearson Vue and found the 1Z0-591 exam without a problem. If I had any desire (and sufficient knowledge in BI), I could have scheduled it. The exam is there. I can't give you a link to that because it requires logging in.
... aren't on-line exams common ?
No, almost all of them are proctored. On-line exams are frankly just too easy to cheat on. The certification would mean very little if most of the exams were available on-line. This is not to imply that you would, but it is certain that many people would take advantage.