Go online! The best sources for any art supplies at the best prices have always been online for me. Start with http://www.dickblick.com because they have great prices, great customer service and amazingly huge selection. Then http://www.aswexpress.com especially for what's on sale.
There are some specialty online places for sumi-e and other Asian art supplies including http://www.acornplanet.com which does carry individual sumi-e ink sticks and the grinding stones as well as complete sets. They also have the colored ink sticks. When you say tablet, I'm not sure if you mean the suzuri stone that you grind an ink stick on or the ink stick itself. Acorn Planet has both.
http://www.orientalartsupply.com also has a wide variety of sumi-e supplies including the ink sticks and suzuri stones. Both of these Asian suppliers also carry the red paste and seal stones for doing a traditional seal signature on your sumi-e paintings.
Other places you could look offline are import stores, art supply stores and craft/art department stores like Hobby Lobby, but I would trust the online suppliers every time for price and quality. It's worth waiting a few days to pay less and get better supplies. Acorn Planet has cheaper ink sticks, but OAS has some really nice ones and both carry good paper for sumi-e too. OAS also has mounting film so that your thin rice paper can be mounted to silk or archival mat boards for display without bubbling, plus instructions on how to use it.
Both of the Asian sites also have good articles and background reading.
Hope this helps!