Acutally, most of these stuff's you can't find in any of these so called stock-gallerys. Why not?
Because these are mostly hand-drawn (in the traditional way or digital) and than edited on computer, where it get's it's final form.
What I would do, is, I would simply draw it, scan it, than vectorize* it.
Postproduction - if bitmap - can be made with anything you like (Photoshop, Corel Photo-Paint, Pixelmator, Gimp etc).
The only border is your fantasy.
* but this is not necessary, since you can scan the original drawing in high-quality 1000 or 1200 dpi bitmap/lineart mode. But don't forget, most graphics will be 300 dpi or less in final mode, when printed.
Here is one of the things, I made in the past:
Original drawn lineart:
Final version:
I also recommend you, you check out some of the blogs and websites of my formers school-mates and friends, especially their illustrations:
http://gyoritibor.blogspot.com/
http://www.szugyigabor.hu/
Here is one of the guys, whom work I really like:
http://www.hydro74.com/rebirth/illustration.html
Also check out these guys:
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Ljubomir Babic
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Jeff Owens
Here is a nice tutorial for the inking process:
http://www.gomediazine.com/tutorials/from-sketch-to-vector-illustration/
A lot of that type of work is either stock, clip, or derivative.
If I needed to do images like those lions, I would go to the library or internet and find something that would fit my needs as a base starting point. I would then trace over the image and refine/simplify it. By the time I was done, it wouldn't look much like the original. Deriving from something that already exists is what designers do all the time. My favorite method is to combine several images (that I often get from old books) to create something total fresh and new.
As you do this kind of work you will begin to build up your own library of graphic assets you can use on other projects. When I'm in between projects, I'll often create/explore image ideas. Those ideas often make it into future work.
These are really good thoughts.
However, one thing to mention is: mostly you cannot use the same thing over and over again for your designs.
Why not? Because soon it becomes boring.