In 
order to cook sour curry proper and fast you should have some "ready to use" curry paste. It is the key of success in any Thai curry.
Ingredients
1. Dried big red chilli : 6 ea
2. Sliced red onion : 3 table spoons
3. Steamed fish meat or shrimp meat : 50 grams
4. Salt : 1 tea spoon
(If you don't want to cook this by yourself, you can buy it in any Thai grocery store.)
Direction.
1. Remove seeds inside that dried big red chilli
2. Soak it in water for a while
3. Grind it in food porcessor with salt
4. Put other ingredients althogether and grind it till fine :D
Storage
After you prepared this paste you can store it in the fridge for months.
Tips.
My mother like to use mackerel fish in tomato sauce in instead of fish meat because she likes it easy (no need to boil, no need to remove fish bones :D).