I couldn't get castor oil in Boots, superdrug or holland and barrett or any where on the high street so got some from akamuti here
If you are lucky enough to find it in boots etc it will probably be cheaper.
I too have combination skin and use approx 20% castor oil to 80% sunflower seed oil (from the kitchen cupboard...)
I don't mix in essential oils, but don't see why you couldn't add lavender, tea tree or any you fancy. I would only add a very small amount to begin with in case of reaction - plus if you are like me I use the oil to remove eye make up and would be wary of usng strong levels of essential oils near my eyes.