For movies, I usually go through The Movie Database. It's what iFlicks defaults to when it tags my movies and most movies have a lot of different posters to choose from. If I can't find something on there, I'll check out Valkae or Get Video Artwork. Both of these are also good for TV shows (although GVA has a much larger selection than Valkae).
For TV shows, if I can't find it on GVA, then I'll hit up The TV Database. They don't have artwork in the 600x600 iTunes format for TV shows, so I'll muck about with the images in Photoshop until I get them to where I like.
And if I can't find what I want on Valkae, MDB or GVA for movies, the Movie Poster Database has some great images that I'll mess around with in Photoshop if need be. Failing that, Google Images.
...I spend too much time on video artwork.
http://bendodsonapps.com/projects/itunes-tv-artwork/