Best place to get CSV files for importing

If you're just trying to add an entire set, as a premium member you don't need a CSV. Just go to the correct set from the Editions page and choose to add 1x of each card from that set. If you don't have some of the cards (such as the basic lands or the "extra" cards from the Planeswalker decks), then it'll be much easier to remove half a dozen cards once you've added the entire set this way.