Clip Art, Copyrights, Etc.

Frequently the big guys (Disney, etc...) will not sue smaller or non-threatening entities over copyright issues for the simple fact that there is enough risk in a suit on their end to make the cost-benefit scales tip. If, for example, DC comics were to go after monkeyman's images and a court ruled against them, they could really be hurting, and even risk losing their ability to protect their IP. Monkeyman's images, for example, aren't hurting them enough to warrant the risk (and sometimes the reference even helps as free advertising). This isn't to say they will never go after smaller uses of their IP. And it isn't advocating using other people's IP just because you don't think they will come after you. It is just an explanation of their motivations on a business end. Using other people's IP does put you at risk, and sometimes they will approach you to leverage a settlement or issue a scary cease-and-desist letter; and if they force you into court, you can get really hurt under current IP laws (particularly the copyright act). So do what you want, but know what your risks are when deciding.

As for your original post, if you like using clip art, check out the open license clip art galleries online (can't remember the urls right now.) These usually contain usable images (although you may have to dig for them) and require little more than a footnote or credit to the creator (although look at the open license documents that these websites provide to see exactly what you are allowed to do).

Additionally, cartoon art, like clip art, does tend to be cheaper than more realistic or fine art to invest in because it is much less time intensive, so you may be able to find some reasonable deals. You can also negotiate with local art students who are looking to build portfolios and get a decent rate, although reliability of art students who also have class work can be spotty.