Creating an 11x17 brochure and require background images. Where can I purchase images of suitable quality/size to use for this purpose?
As Andrew states, Stock image websites are a good resource for things like these. You can find stock sites in any price range, from free (eg. http://www.freeimages.com/, https://unsplash.com/) to outrageously expensive (http://www.gettyimages.com/) and everything in between. Try and google around to find something that fits your taste and wallet.
If it is designed backgrounds you are looking for, any stock site will have lots of them. Try entering 'background graphic' as a search term and you'll find some nice stuff just about anywhere.
For some more offbeat backgrounds, I like using http://www.textures.com/, which has hi-res pictures of all kinds of patterns and structures. It's originally intended for use in 3D environment design to skin the polygons, but I find it a real trove of background graphics.