LaserDiscs

Leopold's Laserdisc page: A great set of information and jumping off points.
Ken Crane's: The biggest post-DVD (post apocolypse?) laser disc collection I've found, and the Criterion Collection's current LD online retailer.
Laser Disc Exchange: Buy laserdiscs on-line
Criterion Collection: They no longer have an online store, but they are still adding things to the collection. This page lists where you can buy them, and have some other useful information.
Pioneer Entertainment: They sell their lasers, and have some great deals.
Image Entertainment: Their recent laserdiscs are listed. Don't know if they retail online.
RPM Records: Buy some laserdiscs (and other items) on-line (not many these days)
C&L Multimedia Distributing: Buy some more laserdiscs on-line