Popular vector shapes and background tiles can be used in many different website layouts. Vector graphics themselves are easy to use because they can be scaled larger or smaller without quality reduction. It just takes a bit of work and some time in a graphics editor, but vector shapes may provide some of the highest quality value to your websites.
When you need to find quality vectors I would think there are plenty of resources online. But some designers are looking for specific releases for their logos, icons, or other various interface features. I want to present just a couple online marketplaces for obtaining the most creative and unique vectors online. Check out some of these resources and see if you can use anything on your next web project.