Sabine, there is a shop called Kolmsperger Handarbeiten near the Stadtmuseum which might have what you're looking for. They are a specialty stitching shop, and have good customer service. Here's a link to their website (in German):
http://www.kolmsperger-handarbeiten.de/ I don't do needlepoint but I do some cross stitch/sampler stitchery, and was a bit disappointed with the small selection here (compared with the US.) However, there are quite a few online mail-order shops in the UK where you can have things shipped to you here without customs charges. Just google "needlepoint UK" or similar to get started.