It took me ages to find correx. I eventually tracked it down from
www.technologysupplies.co.uk They sell 3mm and 4mm in black, white, red, yellow, green and blue. The 3mm cost me £18.75 for 10 1000mm x 600mm sheets (minimum pack size). The 4mm was a little more expensive. You have to phone your order if you are not an account holder (reserved for educational establisments) but they are really helpful and delivery only took two days. I would highly recommend them. They are based in Shropshire but postage was not too expensive, even to Aberdeen.