In my experience it is better to order them or go to one of the many open days the member on this forum do. The reason for this is that the condition of the plants is generally guaranteed to be good.
Most plants I have bought from garden centres don't last long because they have been watered incorrectly. That said if you get them when the order has just come in, before the staff have killed them with tap water, you should be fine.
There are so many people on here that will sell you great plants at good prices you might as well use them :)
Hope this helps