Is it possible to buy Spectrum Analyst for Sonar X3 ?

I don't think so. Zoom is exclusive to the Producer edition. However there are options for spectrum analysis, like Voxengo Span. Also I'm pretty sure the analyzer that used to come with Sonar is still part of the installation, it just doesn't install by default.
 

Voxengo SPAN and Meldaproduction's MAnalyzer are my favorites, and both are free. The latter is part of a free bundle that has gobs of other cool goodies in it too.
 
There are also multi-channel versions of both of these that aren't free but aren't expensive, either.

My two cents...people always say "don't mix with your eyes," but the more senses you can get involved in a process, the better. Devices like spectrum analyzers can be an important part of ear training, like when your ears tell you that something sounds "muddy"...but the spectrum analyzer shows you the mud is somewhere around 300-400Hz. You're now in a better position to correlate EQ settings to what you hear, and dial in the right setting without having to refer to the spectrum analyzer.
 
Another example of ear training is delay. Back in the days of the Echoplex, if you'd asked me how much delay I was hearing, my answer would have been "I dunno." But since the advent of digital delays, I've learned what delay times sound like and can come pretty close to identifying delay times.
 
A final example is modeling with EQ. Give me nine bands of parametric EQ, and I can make guitar A sound like guitar B to a degree of accuracy where a guitarist can't tell the difference in a blind test. But give me a spectrum analyzer, and I can do it in 15 minutes. If I have to do it by ear, it takes a whole lot longer - but I've already gotten much better about getting frequencies "in the ballpark" because of what I've learned from using spectrum analysis.
 
So, while a spectrum analyzer won't necessarily help you create a better mix, it can definitely help train your ears by correlating what you hear to specific physical frequencies and characteristics...and that will help you create a better mix eventually.

Sorry to disagree with Mr. Anderton, but most of the mixing, and especially editing problems that I've witnessed have been the result of people using their eyes instead of their ears.

I will agree that a spectrum analyzer is a fantastic training tool, so is an oscilloscope. But trying to use them to edit or mix misses the point, I think. While I would not give up the many advances that DAWs provide (especially the undo button) I would argue that I can still make a more musical edit, more quickly, with a razor blade. AUdible scrubbing is one probably the only thing I still miss about tape decks.

wst3
Sorry to disagree with Mr. Anderton, but most of the mixing, and especially editing problems that I've witnessed have been the result of people using their eyes instead of their ears.

Read past the first sentence. I was talking about using your eyes for ear training, not mixing, and the benefits of having both senses involved in the ear training process.
 

I would argue that I can still make a more musical edit, more quickly, with a razor blade.

 
Have fun with that...if I never have to do another window splice in a 2" 24-track and use that stupid powder, I will not feel like my life has been diminished in the slightest.

Meh to that, I use an analyzer whilst mixing (ear too, obviously) in a "measure twice, cut one" kind of double-check. Plus sheesh try and ballpark a reference mix against one of your own without one? I'll take the analyzer over ears for that too. 
Making electronic music? Good luck using your ears on that sub bass. 
 
Are you older? Making music for younger folk? Also good luck with just the ears. That's why I've started listening to all that "hippie music" now that I'm a fuddy duddy. That techno business must be made for dogs or something! :-)
 
 

I think IZOTOPE also has an EQ with Analyzer. I will check Fabfilter too.
 
Thank you
 

...wicked
Are you older? Making music for younger folk? Also good luck with just the ears.  

This is so true in my case. After over 30 years in emergency services, dealing with sirens and all types of loud noises, the top end of my hearing is pretty much gone. Some did come back after I came off the street and went behind a desk. But when mixing, I have to rely on my eyes, as my ears simply can't do the job.
 
Also, let this be a lesson to you younger folks out there. Take care of your ears. Once you start to lose it, you can't get it back!