Great big place in the centre of Tsukuba. Seems to stock nearly everything. Prices are usually pretty good.
This store only rents DVDs. Their collection is quite small, so you may not be able to rent the newest movies right away, but you have a fairly good chance of finding the odd old one. Ishimaru has the best prices (usually 300 yen or less) for DVD rental. If you have a DVD player and can wait to see the newest movies, this may be the best shop to use.
Google map in Wikimapia map