Hi there! I'm sorry to hear that you need a replacement nib for your Copic Sketch. Luckily, we have something that will work perfectly! We offer a Medium Broad nib that is made of stiff felt tip, so you can easily draw very thin lines. It's easy to exchange with your existing nib, so you don't have to worry about any complicated installation. Let us know if you have any questions and we'll be more than happy to help you out. Thanks for your interest in our product!