Where do I find a better mirror? I recently bought a telescope, but it doesn't have a very clear image.  Where can I buy a better mirror?
Well, if you buy a car and then you go "this thing doesn't drive well, where do I find a better engine" - then folks are going to ask you questions such as: have you driven stick shift before, do you understand how the clutch operates, etc.

There are many reasons why a telescope does not provide a satisfactory image.

Perhaps the expectations were too high. An amateur telescope can provide quite amazing views, but it's never going to be equal to a large professional telescope in orbit around Earth.

Also in line with the expectations thing, all else being equal, a large telescope will always perform better than a small one. A 6" scope and a 24" scope will provide very different levels of performance. If you have the 6" scope, but you expect 24"-level performance, that's never going to happen.

Perhaps seeing (atmospheric turbulence) was bad during the time when you tested the instrument. For objects in the solar system, seeing is one of the major impediments - it reduces resolution and muddies up the image. It changes in time; sometimes it's good, other times it's bad.

Perhaps light pollution was the problem. DSOs (deep space objects, which have low brightness) will easily get masked by the regular light pollution in cities. It changes with the place; it's bad in cities, it's good far from cities.

Was the telescope collimated? Some telescopes, such as dobsonians (and most scopes with a newtonian OTA) require regular collimation. Many SCTs do too. Performing collimation on such a scope is like changing the oil on your car - you just have to do it periodically.

Was the telescope stored in a warm place (inside the house), then suddenly taken outside in the cold night? The temperature differential will be similar to bad seeing for up to one hour, until the scope and the air take the same temperature.

Was the scope properly focused? In many cases, finding perfect focus is a little tricky. A small turn of the knob could make a big difference.

Can you even reach focus? It's surprising how often people have weird setups where the eyepiece cannot even reach the focal plane of the primary mirror. In that case, no amount of twiddling the focuser will provide a good image.

Is the eyepiece bad? Sometimes mass-produced scopes come with eyepieces that are just not very good. It happens. Trying a known good eyepiece instead should provide proof.

Are the optics very dirty? A layer of dust on the primary will not affect performance in any way that you can perceive, so leave the dust alone. However, if there's a big fat oily smudge on the back side of the eyepiece, then of course performance will be degraded.

Are the optics seated properly in their mirror cells, etc? A primary mirror not sitting flat in its cell will mean the scope will never perform well; this could happen during transport with some cell designs.

Is the frame or the OTA intact? Has it been bumped or otherwise deformed?

Finally, yes, in some cases mass-produced scopes have bad optics. In that case there's nothing you can do except replace something - either the defective mirror or the whole instrument. Keep in mind that a custom mirror will cost more than your whole instrument, if it's high quality.



TLDR

You're not providing any details, so it's hard to say what's going on. Based on experience, I'd say the most likely candidates are:

If your scope is a dobsonian or an SCT, I would start with collimation. Learn how to collimate the scope, and do it regularly. It has a very, very large impact on observations of the Moon and the planets. Do this before you try anything else.

If you're observing DSOs (galaxies, nebulae), maybe light pollution is the reason. Load up the scope in your car and drive about 1 hour away from the city, and try again.

Learn how to focus the instrument. It could be trickier than it seems.

Finally, go to a local astronomy club meetup and try and use an instrument similar to yours. Maybe your expectations are not calibrated properly.