I get paranoid when I don't have a handful of 3x5 cards and a binder clip. The cheaper the better why not bundle them and discount in packs of 1000 or more for volume purchase?
We needed a quantity of index cards for a reasonable price and found them at Staples. Teachers require them for research notes and to make language flashcards. My daughter can keep them organized for the year and they don't take a lot of space in the backpack or locker between classes and study time. The lined format helps her neatly write research notes for English or the Spanish word definitions and English translations or she can easily print large text for flash card use.
My high school son uses index cards all the time for school. Through the years we've found it best to take the new packs of 3"x5" cards to the local copy shop to have them cut in half. He gets double duty out of the 3"x2.5" cards! He uses them for calculus foreign language SAT prep and even to memorize scripture! Once they are cut in half they store more efficiently. Last year (2007) was the first time we saw these "half index cards" on the shelves at Staples but no one was selling them this year and they cost more than the regular ones (go figure!). So we just buy the Staples brand and have them cut before we head home. Having bulk pack cut costs about $1.40... so its worth it.