Liberty, Tower, Virgin Superstore ummm, seriously, just go to Akihabara, you'll find DVD
"DVD wa dohko des kah?"(Where can I find DVD) If they are confused, try pronouncing it Dee-bee-dee
For addresses, the Japanese numbering system is so screwed up that you're better off just wandering. Seriously.
Jeff has it right when he says just go to Akihabara. However, there is no Tower or Virgin Megastores in Akihabara. The best places to find DVD in Akihabara (the v is pronounced "vwee") are at Ishimaru (there are 3 of them in Akihabara) and Disk Union (in Ochanomizu--one stop from Akihabara on the Yellow Line). If you go to Shinjuku you can visit Tower (Shibuya Tower actually is bigger) and HMV. All of these places can be visited in one day because they are close to one another.
In Japan, discounts of 10% are generally given as store coupons good on next or future purchase. That is the way Ishimaru works. The best place in Akihabara is Dynamic Audio. They give 15 % discount up front. No coupons. The Dynamic Audio store is about three buildings to the West of Ishimaru Soft 1. This is on street that intersects with the main street. North East corner at main intersection is Yamagia main store. Nortwest corner is Bank of Tokyo-Mitsubishi. Head West from this intersection about two blocks. Dynamic Audio is on North East corner of next "main" street. HMV and Tower use point card system for discounts. The best HMV is the one in Shibuya. I highly recommend Dynamic Audio.
Thanks for that update on Dynamic audio. Disk Union is where I usually go when I am in Tokyo (they give 10-12% up front the last time I was there). Have you been there before?
Either way, a lot of great places to visit when visitors come to visit.