I have a recipe for making it yourself, it's nice although you might need to add more hot water if you prefer a less lumpy sauce as it does tend to be quite lumpy, I usually just heat it a little longer until it's smooth:
2 tbsp Plain Flour
2 tbsp Vegetable Oil
2 tsp Medium Curry Powder
Small Amount of Garlic (either a clove or a few tsp garlic powder)
3 tsp Chilli Powder
2 tsp Chinese Five Spice
400ml Beef Stock (boiling water)
Put the flour and the curry powder in a bowl and mix together.
Heat oil in a saucepan with garlic and chinese five spice.
Remove from heat and add the curry and plain flour and mix.
Put back onto the heat and add a small amount of the beef stock until it forms a paste. Add Chilli Powder.
Continually adding Beef stock a little at a time. Continue heating until you form a lumpless sauce.
Advise you to cook a few hours in advance if you're using it for chicken curry, leave it to cool down and then slowly stir it into the chicken when it's cooked.