if the door cards are warping due to humidity then there is some leak in the plastic barrier or the interior has some other water leak.
The cavity wax is called body cavity wax - it will be available at almost any autobody paint supply store or even american brand dealerships (GM, Ford, Chrysler etc) -ask them for the spray that is done inside doors and body panels.
if you cannot find it then get a few candles, remove the door card, clean out the door cavity completely, open up all drains and then rub the candle on the door skin from inside - it makes a very good coat of wax but its not very long lasting due to high heat in your country.