Okay, so I've always used Roland's Sound Canvas VA, as I come from the 80s & 90s when I used Sound Canvas exclusively.  But now I want to add better/newer sounds to my arsenal.
 
#1 - Where can I find more sounds (particularly drums and bass)
 
#2 - What's the procedure to make them work in SONAR?  I'll be triggering them via MIDI with a Yamaha keyboard.

You have practically an unlimited number of sources to explore.  All of them, I believe, will be a VST plugin in one form or another (guitar sounds, orchestral, synths).
 
I'm considering dropping the $199 for a synth by U-he, called Zebra v.2.8 because you can download several thousand presets for free from the users who have uploaded sounds to share on the KVR website.  I also happen to like another synth from U-he called Repro.

Zebra2 : https://u-he.com/products/zebra2/
Patch Library : https://u-he.com/community/patchlib/ 

You can explore this subject in depth on Gearslutz under Electronic Music Instruments & Music Production category as a starter over there.

On this site, I've asked specifically about Electric Guitar sounds and got a heap of answers.

U-he is indeed great. All of their plugins. Just top-class.
 
But as for most bang for the buck for the OP, I'd suggest looking at NI Komplete, that covers a massive range of sounds, with thousands and thousands of sounds, effects, etc.... It's the most "komplete" bundle for high quality sounds IMO. They have three bundle sizes, ranging in price from $200 to $1200. 
 
If that's out of the budget, then I'd suggest searching for the best individual VST plugins one-by-one that cover the specific sounds that the OP needs. I'm not too familiar with free plugins, perhaps someone can steer you the right way there. But as for paid plugins, they range in price all over the map of course... 
 
For drum plugins, there are so many, I don't know where to begin... Some of the big developers include: NI (they have several), XLN audio, Toontrack, FXpansion, Steinberg, etc... but this can get niche really quickly... with library expansions, etc... 
 
For bass, really depends on what kind... again, tons of developers, but this can also get niche really quickly. Again, maybe start with NI, and work out from there until you find what you're looking for. The are incredible sample libraries that work with NI Kontakt, for example, but if you want to get into more synthesis, there are tons of amazing VA synths that will do the trick. The previously mentioned U-he makes some of the best VA and modular kinds of synths around.
 
Have fun, try things out, find your muse! Good luck!

Why not download free Sonat Plat, now known as CWE by Band;ab. Has some nice sounds that come with it. Better than a sound canvas for sure.
Also agree that Kontakt has great sounds and easy to use.
 
Funny, I also used a sound canvas in the 80's and 90's...fun stuff...even a yamaha tx81z

For drums, either (for near production ready):
 
EZDrummer by Toontrack (watch out there are like a gazzlion add on kits, but the basics will get you going)
Addictive Drums is pretty good too (they have a gazillion add on kits)
Steven Slate Drums 4 by Steven Slate (but Version 5 may be out soon)
 
 
If you want to and be a full on producer, then for drums:
Superior Drummmer 3 by Toontrack has some good kits, add on either their kits or even use EZDrummer kits
BFD3 (loads of add on its too)
 
For Bass:
Trillion (don't use it but I hear it's the bomb):
Orange Tree Samples (but can use the free Kontakt Player by Native Instruments, IIRC)
Ample Sounds has some nice basses, watch for sales though
Native Instruments has some nice basses to but they require the full version of Kontakt
 
For general sounds: Native Instruments Komplete 11 Ultimate (take out a 2nd mortgage for it).

The OP has Artist which I do believe comes with exactly what CbB has. 
 
I used a similar sound module as my backing band for about 15 years, A Korg 05RW 
It was a huge upgrade to start using Sonar , at the time 8.5, to make my backing tracks. 
I used Session Drummer, True Piano's and SI bass for years. I used TTS-1 for Organs and other stuff. 
 
Now I use Addictive drums, ( came free with Sonar) Ample Bass P lite ( free) And Addictive Keys ( free)  
And then I use Dim Pro and a bunch of Air stuff for organs and whatnot. 
I just redid over 120 backing tracks using that "band" very happy with how it comes through the PA. 
The only thing I ever paid for was the Air Expansion pack which I think was only $50. 
 
There are so many good freebies out there I would defiantly start there. Keep your money in your pocket until you either see a deal or just got to have something. 
I highly recommend starting out with the Ample Bass free. Only bummer is it can't go below E. The paid version does. 
The other incredible freebie is AD Keys. Its the demo and like the Ample bass only plays in the middle of the keyboard. Has never been a problem for my music. 
To start out try the SI drum kit. I've managed to squeeze a OK drum sounds out of it . I like the Natural Kit and put more compression on it. 
 
I see you have a Focusrite interface. Go log in and grab the freebies offered there. That's where I got the Addictive keys from.  
 
The learn how to set up Sonar to use MIDI follow my tutorials you'll find in my signature link. 

Check out all the specials and ads at the bottom of this page, which all have to do with what you are looking for - Sounds and synths.
 
https://www.kvraudio.com/...ce/dialog-by-wave-arts

stratman70
Why not download free Sonat Plat, now known as CWE by Band;ab.

Link please?  I can't seem to find it.  Thanks.

Cactus Music 
I see you have a Focusrite interface. ...

Yeah, that's incorrect - my bad!  For some reason I listed it as Focusrite when it's actually Steinberg.  Never noticed that error until you mentioned it and I went to find the software you mentioned.  Of course I was in the wrong place, lol.  Wow... absolutely NO IDEA why I put it down as Focusrite at the time (two years ago!)