If you search the IAP site, you will find several posts on making refillable Sharpie pens.
And yes, you can purchase Sharpie refills. In fact, Sharpie offers two different types of refills - they are not interchangeable and are designed to fit into different types of housings.
However, the issue in making Sharpie pens is that the refills are designed to screw into a female opening in the housing, and the thread in the housing is not standard. I think a few people have invested in bespoke taps to cut the required threads, but that's an expensive option.