This may not be as helpful as you would like but, having noticed, when faced with replacing a few tiles on a floor which was entirely covered with cork, that material currently available in the UK is only half as thick as the original all I can suggest is that if you or someone you know is going to the Continent they visit a DIY/Building supplies outlet there. The last I knew thicker cork than available in the UK was on sale there (more like 10mm or so), but you do not say what thickness you are looking for nor in what size of pieces - the ones I have seen have been too small to create a notice board without using more than one piece to complete it.