These are some of my favourite sites for all things tarot:
Tarot Association of the British Isles - a great group of people with an active, friendly forum, the Association also offers training and endorsement courses.
Aeclectic - a very active forum, and an informative site, with lots of deck images and reviews.
Tarot Garden - another site that offers lots of deck images, they are a retail site in the U.S.A. and also offer an "Out Of Print" deck search service.
My favourite places for bags and cases:
AniaM - a British retailer, Ania has lots of lovely embroidered tarot bags and is also very happy to customize bags to your specifications!
Baba Store - this site offers tarot bags, but also all kinds of other items with images from Magic Realist Press decks on them. Beautiful and good quality.
Tarot Sulis - another Brit, this website offers lots of patterned material tarot bags, and I especially love the beading on the drawstrings - beautiful and practical...
Tarot Totes - this is an American firm, selling a huge variety of lovely bags. I also especially like their padded cases, great for tarot decks or mobile phones.
And some sites to buy decks:
Alida Store - this is a good place to buy European decks which aren't always available through English speaking shops. They have Lo Scarabeo decks before other outlets, though normally without the accompanying book in English if there is one. They also carry items from German and other European publishers such as AG Müller.
Magic Realist Press - this is the brain child of the creators of a number of beautiful decks. They're also related to Baba Stores, where their images are put on tarot and other bags and items. Worth checking out.
The Tarot Garden - this American retail site is great, with lots of info and deck images. They also offer an "Out Of Print" or hard to find deck search service where you say how much you'd be willing to pay and the condition your willing to accept and they see if they can find it for you! Or you can just haunt ebay