Field equipment can be difficult to procure for an aid worker, mostly because it's so fragmented across different websites and store locations. There's no "Cabela's Field Economist Warehouse" (though I would love to see one...) so I'm going to try and maintain a guide here to the best sites to search for the products you're likely to need.
Rugged cases for laptops, medical equipment, etc.: www.pelican.com
iPhone cases (waterproof): www.lifeproof.com
Waterproof paper: www.riteintherain.com
Waterproof bags: