if you put the plant name in the search section on the Garden centre Tab you may find one there.

The gardening express website advertises them - if you copy and paste the following into your browser hopefully you will see it.

http://www.gardeningexpress.co.uk/ProductDetails.asp?ProductID=1247

You can buy seeds for a cheaper option:

http://www.thegardencentregroup.co.uk/Penstemon-True-Blue---flower-seeds-(50-seeds)/0820000436,default,pd.html