There are many online clipart services that provide excellent clipart for rubber stamping; the best images are from services that are subscription based that charge a yearly or per image fee. If this does not suit and you really want to save money you should find out which service your rubber stamp manufacturer belongs to, it costs nothing to search through the clipart service that he belongs to as he will be the person downloading the images. This bit of lateral thinking and finding the right manufacturer will save you lots of money because you will never have to pay for the clipart. Clipart.com is one of the best services that some manufacturers subscribe to; I estimate that over 3 million images at Clipart.com are suitable for stamp production and they are fine with manufacturers using the images for rubber stamps.
The type of clipart that I use is important for all of the different effects that I'm creating. It needs to be a vector drawing for sure, as non-vector drawings are bitmaps that don't resize nicely... they'll just get all bumpy and not look good. Also, I'm drawn to simply line art because if it's too complex sometimes it doesn't look as good in the different sizes. It's also good if it's a single color, usually black, because then I can change it to whatever I want quite easily just using simple graphics software. Just remember, simple is better, is cleaner, and usually creates a more sophisicated look.
Kid's art is great for use for kid's rooms, grandparent scrapbooks, and kid's scrapbooks. These pictures can be used as backgrounds, as focus items, or as embellishments.