NATIONAL GEOGRAPHIC TELESCOPE NG76AZ where can i get a solar filter and additional lenses etc for above 76mm telescope

Do NOT attempt to look at the Sun through a solar filter! These can crack from the heat and you will be blinded instantly.

Most telescopes use a standard fitting eyepiece with a 1.25 inch outside diameter barrel. You can measure the mount where the eyepiece goes to make sure that yours is this size, then search on eBay or Amazon for "1.25 eyepiece" and you will find many available. None of these will be made by National Geographic, but any make will fit.

The other variable will be the focal length of the eyepiece, which is what determines it's power. The power of the scope will be the focal length of the main objective divided by the focal length of the eyepiece, so a 9mm eyepiece will give a higher magnification (and be dimmer and harder to focus and find objects) than a 20mm eyepiece. It is usual to have two or three different focal length eyepieces for viewing different objects.

You will find a large variation in prices, which is partly due to the difference in quality and type of construction of the eyepieces. Kellner eyepieces are simple and cheap, whereas Possl or orthoscopic ones are more complex and cost more. There are reasons why people will pay more for the better eyepieces. Avoid eyepieces marked with the letters "H" or "SR" before the focal length.