Telescope eyepieces are interchangeable. Most scopes come with two or three, of different focal lengths (to give different magnifications), and enthusiasts frequently buy others, either to get a wider range of focal lengths or to get better quality.
I do not know what you mean by "does not even belong to the telescope". Most eyepieces are a standard size, with a barrel of 1.25 inches outside
diameter. If your eyepiece mount is that size, you can use any make of
eyepiece that fits. The Tasco brand name is not associated with high quality eyepieces, so
if the one you have is another make then that is not a problem in
itself.
You can find many eyepieces for sale on Amazon or eBay by searching for "eyepiece 1.25". You may want to familiarise yourself with eyepiece issues by visiting
THIS SITE, which has useful basic information.
