Tupperware is FANTASTIC for keeping your groceries fresh, and cost 3-10 times as much as the competition. So if you're not in desperate need to keep your guacamole green, go for another cheap brand.
Looking in any $0.99 store, they always have some sort of plastic food containers. Typing "plastic food containers" into Ebay lots of hits, many of them for IKEA stuff, so maybe Ikea is worth a look.
My favorites at tne moment at small white boxes used by farmacies to ship bottles of injection fluid in. Every time I'm in a farmacy or a doctors office (having an allergic kid adds to the frequency) I ask for them. Usually the whitecoats aren't to eager about giving it away, but when I explain that I need it to keep some order in all my robot parts, they realize that I'm a harmless nerd and give me plenty.
They come in different sizes, but the smallest ones fits 13 AA batteries stacked tight.