Have you looked at the 'Garden Centre' tab on this page? You might find a stockist via that route.

Or, you could use the RHS website at

www.rhs.org.uk

Go to the 'Plantfinder', type the plant in, and you should get a list of stockists.

Finally, there's good old 'google', where you should find Nurseries who will sell it.

These should be quite easy to buy as they are a common house plant. You may need to try a large g centre with a good house plant section. If they don't have any ask why not!! Also, try Homebase, B & Q etc. or a florist.