Are the apostles the final authority on interpreting the scriptures in the LDS church and how can I get their interpretations of specific passages? I heard a lesson on Isaiah in Sunday school and the manual gave what was to me a surprising interpretation of a scripture. I'm trying to understand the scriptures in Isaiah, (praying included). Having someone actually tell me exactly what it means (assuming the excerpt from the manual is approved by the apostles) like that is a dream come true.

I remember hearing in seminary that one of the roles of the apostles is to interpret the scriptures. Is that right? Are they the final authority? If so, is there some book or reference guide that I could have? Especially, where can I get their authoritative interpretation of Isaiah (because the Sunday school manual only has a little excerpt)?
In a nutshell, yes.


  20 Knowing this first, that no prophecy of the scripture is of any
  private interpretation.
  
  21 For the prophecy came not in old time by the will of man: but holy
  men of God spake as they were moved by the Holy Ghost. (2 Peter 1:20-21)


The interpretation of scripture is the number one reason why there are multiple sects and denominations of christianity instead of just one. 

Isaiah is particularly notorious for being the source of many personal interpretations, as Isaiah delivered his words much in the same manner as Christ delivered the parables–the message is hidden from those that are not spiritually prepared to receive it. 

It's my opinion that your teacher erred in one of two ways: either he offered his personal interpretation of the scripture, or he revealed doctrines that were too "meaty" (see D&amp;C 19:22) to a class without first weening them off the milk.

Regardless of whichever one it was, as with all teachings and direction we receive, it is up to us to take the matter to thoughtful study and prayer to discover the truth of all things. If you find that the words of the prophets conflict with what was shared with you, then you can rest assured that your teacher was merely sharing his personal interpretation.

Read Lesson 8 from the Scripture Study-The Power of the Word Teacher Manual: Prophets Interpret Scripture for more about interpreting scripture.