I don't know if they are the cheapest but generally Corfe's have normally proven to have a fair price for stuff like this in my experience.
Corfe Depot/sales in Hooton and sales in Prenton by Rightway/Half Way House.
I've heard Bromborough Paints & Building Supplies are cheap but not able to ratify.
Only other name that springs to mind is Darlington's but I've not dealt with them since they moved.
In a time of universal deceit - telling the truth is a revolutionary act. George Orwell
When the debate is lost, slander becomes the tool of the loser. Socrates