I bought a fab one the other day from http://www.madefrommarble.co.uk/products.php?category=2&subcategory=35
Bought the light coloured version 38x38 cm and its simply wonderful - keeps the pastry so cool.
Happy pastry making!
I bought mine from amazon.com in 2006 - here's the link: http://www.amazon.com/RSVP-International-18x18-Marble-Pastry/dp/B0000DE6ZA
$60.00 - free shipping (*.*) It's wonderful when working with chocolates, keeps them cool.
Read the 17 customer reviews - to see if it's what you are looking for.