National Geographic telescopes are readily available from a well known catalogue shop. The problem is that some of their eyepieces are not threaded to accept astronomical 1.25″ filters. This is not a problem if you don,t require additional filters. But obviously is, if you do want to upgrade. A solution is to upgrade to branded eyepieces that are threaded correctly.
For a great selection of telescopes and eyepieces, visit Northern Optics