Don't confuse buying pretty boxes with organizing. Don't buy a single container before digging into your office and doing the real work. Overbuying containers, fancy boxes, and other office supplies can be a distraction from doing the nitty gritty heavy mental lifting that will keep your office in shape. Yes, you're likely to use containers and perhaps a few new ones during the process, but resist the urge to buy them before starting to sort, toss, and group like with like. Otherwise, you run the risk of the containers becoming part of the problem.
One of the first things I do with clients when organizing their office is group like items. A grouping I almost always create is unused and underutilized containers. People are usually shocked at how many they've accumulated. By the time we're done weeding through what to keep, what to toss, and where to store things, the pile is usually still there and my client realizes that they never needed those boxes in the first place. Forever empty they stay, and oftentimes most don't make the cut. Don't fool yourself: Buying boxes is not organizing. Making tough decisions and letting things go is organizing.