I find bento making supplies in many places. Here are some of my favorites:
Japantown in San Francisco There are so many amazing shops to source bento supplies, cooking supplies, fun kawaii items, and more. They have an Ichi Ban and a Sanrio store. I spend way too much money every time I visit Japantown, ha ha!
Bento USA This is a great shop for bento specific items like picks, bento boxes, little containers, and things you didn't know you needed.
Lunchbots I super duper love my Lunchbots lunch box. It is stainless steel, has a tight fitting lid, and has three great sized compartments. I use it all of the time. There are so many wonderful lunch boxes on this site.
I also love to shop at kitchen stores like Sur la Table and locally, Hardisty's Homewares, for bento supplies. Target's dollar bins are pretty great for little containers, and I love bakery supply stores. So many cute picks!