You have a few different questions here which I will try to answer in order.
There is no simple replacement for prophets because the era of prophecy has ended. The role of the rabbi straddles many different biblical "positions" including judge, advisor, and interpreter of law.
I am not sure what you mean by "find truth." If you mean a finding of fact in certain legal cases, we can go to a rabbinic court. In other cases, we go to a civil/criminal court. In some, we read the texts and speak with each other about understandings of the text. Or we seek guidance from those who have studied texts and who can help us understand. We might ask for the truth of the application of law and for that we would go to someone who is of the knowledge and experience and authority to establish a supported and persuasive opinion.
The requirements for a rabbi depend on what you mean by "rabbi." In the olden days, the most basic form of "rabbi" was someone who served as a community leader and teacher. But there are other levels of ordination, with each one requiring specific study and testing on particular areas of law. But simple ordination does not confer on anyone the magical ability to state infallibly "the truth."
An addendum as the question has slightly shifted -- re: why are there no prophets today. From the chabad website:
Nevertheless, the principle that "G-d communicates to mankind through prophecy" remains a foundation of the Jewish faith. A lesser form of prophecy, known as ruach hakodesh (divine inspiration), remains the province of the tzaddikim, the righteous men and women of all generations. According to tradition, one of the greatest prophets, Elijah, never died, and will herald the coming of the Moshiach. Moshiach himself is a prophet ("approaching the prophecy of Moses" according to Maimonides), and in the messianic era, prophecy will become a universal phenomenon -- in the words of the prophet Joel, "And it shall come to pass afterwards that I will pour out My spirit upon all flesh, and your sons and daughters shall prophecy; your elders shall dream dreams, your young men shall see visions." And in a letter to the Jews of Yemen, Maimonides recounts an age-old tradition that "shortly before the messianic era, prophecy will return to the Jewish people."
and this site which is, I believe, too long to copy and paste.