Hi, I am an Independent Advisor.
Let me help you with this issue.
Unfortunately, you cannot buy a replacement number pad for the Microsoft Sculpt Ergonomic Keyboard (model 5KV-00001) separately. The number pad is designed to work exclusively with the set it originally came with, and Microsoft does not sell individual components for this model. Even if you could find a second-hand number pad, it would likely not pair correctly with your existing keyboard due to how these wireless devices are manufactured and paired at the factory.
You may need to purchase a new Microsoft Sculpt Ergonomic Keyboard set or consider getting a standalone number pad that works with your setup.