I have only been in Taiwan for 4 months but have already come to the conclusion that every dry, powdery substance has to be put in some kind of air tight container. So…I am looking for a source of cheap or free plastic containers. By free I mean that if I buy some product, the product usually comes packed in a reusable plastic container - just as margarine used to.
Buy some freezer-weight ziplocks (any grocery, Costco, etc.), and some dessicant packets (any DIY baking shop; see sticky in Food section). Inexpensive (e.g. 8-10NT) plastic boxes that seal area also available at the latter.
I have a collection of Lock n’ Lock containers precisely for that reason. If it is not mold, then it’s ants/roaches/bugs (especially rice) and then, as Dragonbones said, use secant packets.