For most of my painting on paper I use Gouache, a water based medium.
I like gouache because of it's intense and bright colours, it is very easy to use and to clean afterwards, you can create delicate transparencies just like with watercolours and very intense opaque colours similar to the ones you can with acrylics. I mainly use Windsor and Newton, thought I like Schmincke too.
I use porcelain paint trays to mix my colours, the surface for mixing is smooth, the paint remains fresh for longer without drying and they are very easy to clean afterwards.
I usually go to the big Cass arts in Islington (they are usually cheap and always have sales on) or Atlantis in east London to buy all my materials.