On www.buntpapier.org is a list of the main European collections (listet under Sammlungen). The site is in German only for the time being and constantly being updated.
I've been to see the V&A collection a few years ago. It is only accessible if the curator takes you by the hand and stays with you all the time as it is housed in the deepest depths of the magazine of this rabbit warren of an old building. Problem is, there is no curator, the collection has been 'adopted' by one of the others who has to find the time and energy for decorated papers somewhere in her own busy schedule.