4Musicals - wide variety of Cats merchandise.
Cats Video Merchandise
Decca Broadway - sells soundtracks (thanks, Yi-Peng!).
Dress Circle - lots of Cats stuff (thanks, Yi-Peng!)!
Ebay - various items up for auction. Check often for new items.
The Jellicle Mall - bunch of items for sale!
Jellicle Transformation - order handmade wigs or tails!
Moonstruck Drama Bookstore - several theatre-related items.
The Music Stand
Muzic Depot - thanks, Yi-Peng! Has soundtracks.
Sheet-Music.Com - buy some sheet music.
Theatre.com Shop - thanks, Sam!

The Really Useful Group - find performance dates.
Telecharge - thanks, Walter!
Ticketmaster - search under Cats to get dates or tickets.
Tickets.com - get tickets, dates, and more.