There is an app on the iPad that is pretty good - 53. Basically it is an digital sketch book with all sorts of realistic pens, pencil, paintbrushes etc. They released a pretty nifty stylus for it not long ago to make drawing with the app a lot better. When you use the pencil/stylus it allows you to rest your palm on the screen without it recognising it as a ‘touch’ - so it allows a natural drawing position. There are a whole lot of other features - if you want to check it out, go here:
