These are sites that I like to visit:

www.moviesunlimited.com (good catalog house with very good customer service)

www.animeigo.com (also Samurai Cinema at their site)  I haven't purchased off of the AnimEigo site itself, but I have bought their products in stores.  Their production quality is excellent.  They listen to their customer base and actually respond to your e-mails!

www.imdb.com The Internet Movie database is a good resource with many consumer comments.  I reference their site for research and in my reviews.

www.advfilms.com They have an excellent product, but they do fulfillment through a third party and their customer service has been so-so at best, so I prefer to buy in stores.

www.dvdempire.com John Nguyen recommends them on price.

www.viewaskew.com Check up on the activities of director Kevin Smith (Clerks, Chasing Amy).