Its very easy, best place to go is probably the DVLA website, find the plate you want, buy it and they will send you a certificate of entitlement, you then need to go to one of their offices and register the plate to the car and thats it.
Hope this helps. You can look at the new style ones too, but be warned you can't buy one that makes your car appear newer than it is, ie. TR06 PHY would be perfect for you but not allowed and probably about a squillion pounds anyway.