So what are we supposed to use when we live in a very humid area if the plastic buckets are not recommended?
Good question. Given your conditions, the cheapest and easiest route would be to put your grains and beans into PETE bottles (and then store the PETE bottles in bins, boxes or an area that restricts light) . A lot of the food, water and juice that you buy at the supermarket come in these PETE bottles. Many of those large blue-colored square filtered-water bottles are PETE plastic and would be stackable. Just clean the bottle out and pour your grains or beans in using a funnel. Add an oxygen absorber to kill any bugs.
Here is some more information about PETE bottles (and how to make sure they are air-tight) http://providentliving.org/pfw/mult...ions_v4_pdf.pdf
You can also repack grains and beans into foil pouches, but they are more expensive and not as readily available (but are more light proof than PETE bottles). Some retailers sell "super-pails" which is a bucket with a foil pouch liner inside. These are more convenient for large quantities, but are also more expensive. You can do the same thing yourself if you have access to a pouch sealer.
#10 cans tend to rust in high humidity. Buckets also aren't recommended in moist/humid areas, but you can use them for storing food that you were going to use within a couple of years (more short-term than long-term).
Here is an additional link (it includes instructions for storing cans, pouches, buckets and bottles): http://providentliving.org/content/display/0,11666,7532-1-4063-1,00.html