Most of the sign supply shops will have various "clip art" collections you can buy, there are also sites online where you can buy clip art by the image or on a subscription basis.
having said that though, you have to be careful when it comes to copyright on things such as logos, you can't recreate logos and such without getting permission from the companies themselves ( you won't get permission from most of them, it's most likely not worth trying) some people think that just because the cd they bought has the Harley Davidson logo you can cut Harley decals, you can't.
there were a few local people around me who had their "business" shut down and their equipment seized because they were selling Harley and Disney decals at the swap meet.