A good source for DVD box covers is dvdempire.com. Find the title you are looking for and click on the image to get the larger version. You can then drag this directly into the Artwork field in iTunes. Of course this is DVD box covers, not movie posters, in case you are interested in these as well.
For tagging, I just recently started looking at
MetaX. I am not sure it will find a permanent home in my workflow, but it does pseudo-automatically lookup information from Amazon, and it has a Movie Poster and IMDB link integrated into its toolbar.