Since there is not a quilting tradition in Italy there aren't many shops. I go to a couple in Milan and one over the border in Switzerland but they are expensive. We pay from 18 to 20 Euro per meter! Quilting fabric is not produced here. Most is imported from the USA or Japan, some from England.
Hope you enjoy your trip. There is so much to see in all three cities and they are so different. I love Rome and Florence but Venice is magic!
Pippa