I usually hit
www.Bestbuy.com or
www.deepdiscountdvd.com
Bestbuy is usually good for new releases, because they sometimes cut prices on release week. Deepdiscountdvd is just good in general. Both sites have free shipping. Matrix at Deepdiscountdvd is
$14.39, likely the cheapest you'll find it anywhere for new.
www.Pricegrabber.com is a good price search engine for non-tech stuff.