I've never seen cooked quinoa in the shops either. But it's easy to cook, and I use the method from 'Supergrains' by Chrissy Freer:
1 cup/200g quinoa
2 cups/500ml water
(Yields 3 cups/555g)
Rinse quinoa in sieve for 30 seconds. Bring water and quinoa to boil over medium heat, cover, reduce heat to low and simmer for 12 minutes or grains turn translucent and uncurl, and have an al dente texture. Red and black varietes may need a few extra minutes. For a slightly nuttier flavour, toast the quinoa in a hot dry pan for 1-2 minutes, stirring, prior to cooking.