How can I buy a Pantone Colour in paint if it is not part of the pre mixed range?

You will need to purchase a Pantone book or a Replacement Page for a Pantone book or a fabric swatch (depending on the Pantone range your colour is from) and take it to a paint wholesaler who can mix paint to a specific colour. VeriVide can sell you the correct colour sample to provide the standard for paint mixing.