Hello,
Can anyone advise me on what instrument samples are available for sonar x1. I am looking for Drums, Bass, Guitar, Piano in fact as many different instrument sounds as possible. I am looking for sample packages that are easy to work with, and give a good quality studio sound. Any advise and recommendations would be greatly appreciated. Thanks.

Dimension Pro which is part of Sonar X1 is rather nice, good variety of sounds across the board, and the built in Z3ta synth is very nice as well.
There are SO many VST instruments out there these days it's just best to take a listen to the demo's and see what you like.  A few of my favorites are:

Steinberg Mach5 3
Spectrasonics Omnisphere
Native Instruments Komplete

You should already have a decent rack of good sounds for this in Sonar Producer. Just take a look at Dimension Pro, Studio Instruments and Session Drummer.

If you're after something REALLY good you should look at Kontakt, the East West Quantum Leap stuff or other high end sample libraries.

Well..  If you want something simple..  like for more drums and session drummer, than grab some of the platinum samples add-ons in the cakewalk store..

as for bass.. read this thread..  http://forum.cakewalk.com/tm.aspx?m=2620086

as for piano and guitar, you already have some links for that..  but you know..  native instruments has some add-ons for kontakt, and ewql, and you've got a lot of piano in sonar already...

I believe cakewalk has some guitar expansions for rapture in the cakewalk shop..  you might want to google applied acoustic systems..

The real question is do you have Essentials, Studio - which comes with Rapture LE...which is decent. Or Producer which comes with Session Drummer, Dimension Pro, all the bells and whistles.... I suggest - if you don't have Producer upgrade and play around with the dimension pro sounds, they are great. If you already have producer, you'll have to look outside to Digital Sound Factory (which sells through the sonar store). E-mu proteus - through sonar store.... Native Instruments (it's website). Google search various sound files that you are looking for. Most reputable companies provide sound samples to give you a taste of what you are looking into. I've have good luck with Beta Monkey for drum files (audio) and groove monkee (midi for session drummer)... apparently primates are good at percussion :)

Also, there are lots of sample libraries that come with Kontakt Player, if you don't want to spend money on Kontakt.

Acoustic Guitars:  RealGuitar (MusicLab)
Piano:                  Ivory II Grands (Synthogy)
Electric Guitar:     Option 1: RealStrat and RealLPC (MusicLab).  Use with GuitarRig or Amplitube
                             Option 2: Electr6ity (Vir2)  I like MusicLab WAY better, but the Vir2 thing is less expensive, has more guitars, and a simple amp simulator.
Electric Piano:       I use the SI-Electric Piano that came with Sonar a lot.
Bass:                    I use the SI-Bass that came with Sonar (or an external synth).  With this, be careful with some of the finger bass samples--they have fret buzz.
Drums:                 Session Drummer 3 that came with Sonar.  I have bought some extra kits.
 
I also have Vir2's Acoustic Legends HD for acoustic guitar, banjo, mandolin, and ukelele.  It is not nearly as flexible for strumming as RealGuitar, but is good for adding a second strummed guitar when the chords are not complex.  It is good for solo guitar lines or finger picking, and I use the mandolin a lot. 
 
I also use some guitar, bass, and drum sounds from EastWest's FabFour, but they would not be your all-around bread-and-butter samples.  One of the best samples in FabFour is an acoustic guitar sound from The Beatles "Blackbird." 
 
If you buy Kontakt 5 you will get a lot of instruments and they will mostly be good.  I personally prefer to pick a smaller number of the world-class sample libraries instead of one pakcage with 1,000 sounds.  My reasoning is that I will have piano, bass, drums, and guitar in almost every song and they will be out front.  If the rhythm section is not good, nothing can save the track. 
 
If you ask about orchestra, I will give you a really long answer.

There are a multitude of freebies and paying ones out there, everyone has thee own personal choice.

Mine ?

Yellow Instruments Free for Bass, Piano = TT1, Guitar, don't use, I direct inject my Digitech rp500, Drums = SD, EZ, BFD ....... the list is long, according to what you want.

Cheers,
Jerry

I am looking for Drums, Bass, Guitar, Piano

X1 comes with True Pianos, Studio Instruments Bass, Session Drummer 3 and Studio Instruments Drums and there are many guitar samples in Dimension Pro.  There is also Studio Instruments Strings and many other synths including Z3TA, Pentagon I, DropZone and TTS-1 (General MIDI synth), SFZ (SoundFont Player). There's no shortage of sounds and tons more available for free or for pay. I have free MiniMoog, ARP 2600, Rhodes Piano, B3 Organ, etc.