Some third-party lens makers sell mirror or, as they’re more accurately known, catadioptric lenses. Such lenses use a pair of mirrors to fold the light path in half, in addition to containing regular glass elements. The advantages are that mirror lenses can be made fairly cheaply and will also be shorter and lighter than all-glass (refracting) lenses of the same focal length. And mirror lenses with focal lengths of 500mm to 1000mm are not uncommon. Russian makers produce a lot of these lenses.

Unfortunately mirror lenses have a number of drawbacks. First of all, they’re manual-focus only. Second, they are optically really slow lenses - usually around f/8 or so. Third, they don’t have aperture diaphragms, so the only way to adjust the exposure is to adjust the shutter speed, the film speed/digital ISO or to put a neutral density (darkening) filter on the lens. Fourth, the smaller of the two internal mirrors blocks the light path somewhat, resulting in rings or doughnuts appearing around bright highlights in out of focus areas. This effect, a form of bokeh, can be visually very distracting. And fifth, they tend optically not to be of the highest quality - you’re not going to get National Geographic-quality bird photos using them.

So, while such lenses are attractive if you’re on a budget, they do have many limitations associated with them. You’re probably better off buying a used manual-focus refracting (non-mirror) lens and adapting it to your camera, or buying a manual-focus body. As noted above, photography can be a really expensive endeavour.