I use these cabinets. I have 4 of these in my shop.
http://www.sevilleclassics.com/storage_cabinet_ultra_hd
They are available at Sam's Club for $209. The have large casters for easy movement around your shop and you can buy additional shelves through the Seville website. They are easy to assemble, they have a magnet that keeps the doors closed while unlocked and the shelves are really made well and sturdy. They make macthing wall cabinets and tool boxes as well.