A solar filter for a telescope is a completely different object from a pair of eclipse glasses, and the difference is where it sits. It goes over the front of the instrument, so the sunlight is cut before the optics concentrate it, which is the only arrangement that is safe at any aperture.
Front of the tube, never the eyepiece
Old telescopes sometimes came with a small dark filter that screwed into the eyepiece. That design concentrates the full aperture of sunlight onto a piece of glass a centimetre across, and it cracks. If you buy a second-hand instrument that has one, throw it away rather than keep it as a curiosity: it looks exactly like the safe kind to anybody who does not know.
Full aperture and what it costs you
A full-aperture filter covers the whole front of the instrument, so you keep all the resolution the aperture can give. Some filters cover only part of the opening, which reduces the heat load and the resolution together. For a small instrument full aperture is the sensible default; for a large one, a reduced aperture is a legitimate way to cut both heat and cost.
Film or glass, and what each shows
Coated polymer film is light, cheap and gives a neutral or slightly blue sun with excellent contrast on sunspots. Coated glass is heavier, more expensive, more durable and usually gives a yellow or orange image that many people prefer aesthetically. Optically the film is at least as good; the glass mostly buys robustness. Neither shows prominences, which need a hydrogen alpha instrument.
Questions people ask about telescope
Can I use eclipse glasses on a telescope?
Never. The instrument concentrates the light and destroys the filter in a fraction of a second. A telescope needs a filter made to fit over its front aperture.
How much does a solar filter cost?
Far less than the instrument it protects, and much less than a dedicated solar telescope: the cheapest hydrogen alpha instrument in this index prints $995.00 for 40mm, while a white light filter is a small fraction of that.
How do I know a filter is safe?
Buy from a supplier on the American Astronomical Society's checked list or a named, traceable manufacturer, and inspect the filter surface for pinholes against a bright lamp before every use.