Depending what you want it's worth checking out general office supply places for stationary storage solutions. I vaguely remember getting good deals from somewhere called euroffice in the past, though recently I've tended to default to Staples or, if I'm feeling really lazy, Amazon.

For slightly more general storage solutions I love Really Useful Boxes (google Really Useful Boxes) with a passion, but they are not the cheapest stackable boxes on the market - they sometimes have special offers on though so it is worth keeping an eye out.