In the UK, and in Australia, you can buy Coriander as a fairly young plant in a pot, and I used to just use the leaves of this, but its better to use the whole plant, leaves, stalks and roots, just wash the soil off. Some south east asian recipes I find ask explicitly for Coriander root, and I havn't seen this on sale. Normally would just blitz a whole plant. Or you can buy coriander seeds in a jar (or ground if you don't have a spice grinder), these obviously come from a much more mature plant. The recipes I use (which mostly come from British of Australian TV chefs), if they are of south east Asian origin are far less likely to use the seeds than the young plant. If they are of Indian origin they may use the seeds as well as the plant. In Spain (specifically the Canary Islands where I lived for a while) you are more likely to use the leaves and stalks, but seeds are used.