This place has some cute stuff for pretty cheap:http://www.ichibankanusa.com/s.nl/sc.1/.f
I know that there was a time when you could get Hello Kitty bentos at Dollar Tree stores, but I doubt they still have them.
Another option is swapping. There is a Bento Swap group on Flickr where you can swap bento items. There were even times when I've swapped non-bento stuff for bento things simpy based on what the person likes or can't find where they live.