Quite a few Asian and Vietnamese stores stock them or you can pick them up at farmers markets in most cities. Some stores have a huge mark up on them so be careful and if you have a few stockists in your area, shop around for the best price. I picked mine up at the PCYC markets in Toowoomba for $2.50/kg and I've heard they're a little cheaper at the Brisbane Markets.
Green cooking banana's or plantains (3 kg of banana's makes about 1 kg of flour)
Peel banana's. (to assist in the peeling, you can boil or steam your unpeeled banana's on the stove to loosen the skin or you can use a knife to cut off the skin). Cut banana's into thin slices and place on baking trays and dry in a low oven for 2-3 hours. You can also dry them in a dehydrator or sun dry them in the sun. The banana's will be quite light in colour when they're ready. Place banana in a high speed blender or thermomix and grind into a fine flour. If you find your banana's are still sticky and aren't grinding, remove from blender, spread on tray and leave to dry for longer. Mill again, sift and store in a dry, airtight container.
Homemade Banana Flour Apple Tea Cake
1 1/3 cup of homemade banana flour
3/4 cup Rapadura Sugar
1 teaspoon baking powder
1/2 tablespoon cinnamon
1 cup of macadamia, light olive or any other neutral flavoured oil
1 cup milk
2 Large pink lady apples cut into thin slices
Preheat oven to 160 degrees Celcius and grease a round cake tin with oil or butter. Sift dry ingredients into a bowl. Whisk oil,eggs and milk together and pour into the dry ingredients. Mix well until combined. Pour half of the mixture into the cake tin, top with apple slices and pour the other half of the mixture over the top. Spread mixture evenly over the apple until smooth and all apples are covered. Cook in the oven for 45 minutes, turn of heat and leave in the oven for another 5-10 minutes. Remove from oven and leave to cool for 20 minutes before turning onto a rack.