Curtain - lining fabric

I want both sheers and drapes on our windows. However, because I cannot decide on the curtain colour just yet, I am thinking of just buying the lining fabric for now (as suggested by the lovely KL). I will then try and see if Mr B's mum is able to sort out the sewing part :)

So what do I need to look for when buying lining fabric? Is Spotlight the best place? How much should I be paying per sqm?

B