I've bought all of my bento supplies on Ebay, so I'd love to share the sellers I used, because I had absolutely fantastic experiences with them. These are all located in the US, which was a high priority for me since it meant lower shipping. I was also very pleased with the prices of all the individual items I purchased.
Bento Mama is my first and favorite. The website says that the shipping for each additional item is $1, but since I bought little items that could fit in the box along with the bento, there was no extra charge. I was really happy about that since most other Ebay sellers would charge that extra dollar "on principle."
Jstuffsale was my second bento store. Again, my supplies shipped quickly with reasonable prices, and I also received a free oshibori with my first purchase. The "I hope you are pleased with your purchase" e-mail was personal and polite. I liked that touch. I also thought the prices here were great.
I bought a gift for a friend at Bento Obento. The price and shipping were good, and it was sent with a free sauce container shaped like a piggy (I'm a sucker for cute stuff like that). The "$5 bento bargain bin" is also great for bento-ing on a budget.