Coriander roots are notoriously difficult to come by (even in well stocked Oriental supermarkets), as in the UK coriander is sold rootless. This isn't much good for budding Thai cooks!!
To get the same intense coriander flavour you would get from one root, use ten stems stripped of the leaves and pound to a paste in a paste and mortar. Use in the same way as you would have done with the root.