I use 1mm and 2mm Greyboard from Cork Art Supplies. Supplied in A1 size sheets. They have what one could describe as 'the usual cheaper on the website' prices. Though not by too much I would add. There are better discounts if ordering batches. Links to the 1mm and 2mm as follows: http://corkartsupplies.com/Grey-Board-1mm-A1-G00382 http://corkartsupplies.com/Grey-Board-2mm-A1%5E-G00383. I've found the delivery service is fast and efficient. Charge is €4.95 anywhere in the country at the moment. There is a minimum order value of €15 incl VAT.
I've no affiliation, just a happy customer.
Mike