Team Fortress 2 store. The Mann Co. is offering the "Humanitarian's Hachimaki" for $7.99, the "Benefactor's Kanmuri" for $19.99 and the honorable "Magnanimous Monarch" for the big relief spender at $99.99. The hats can be equipped by all classes, but can't be traded nor used in crafting.
The items will be available until April 6 and all proceeds will be donated to the American Red Cross' Japanese disaster relief fund. If you don't like hats, the Red Cross has a handy site to which you can donate directly.