I can't find the magic mirror

Magic Mirror usually appears in Gold Chests in the underground. Gold chests also have a higher spawn rate inside underground houses. If you have yet to find a magic mirror, you may simply be having bad luck, since the chests' contents are random.

The quickest way to get a magic mirror may be to roam the undergound and loot all chests you may find and hope for a magic mirror to be there.

I typically use a map viewer program such as MoreTerra to find particular items.
This program can help you find ores, specific items in chests, or particular map features. Depending on how much you know, this may contain spoilers about upcoming features of the map that you may want to discover for yourself.

Depending on how you feel about third-party programs and map viewers, this may be considered a form of "cheating". Personally, after 10 hours without a magic mirror, I would use a map viewer, but that's my patience level.

Gamer15's response is outdated. This can sometimes happen pre-1.2, but not any longer. Chests in the Ocean only spawn with specific items; the Magic Mirror is not included in the loot table.

I would suggest what others have said, just generate a new world. It'll be easier to find one that way.

I've usually found a magic mirror in water chests deep in the ocean.

I think you should just run around for 10-15 minutes on a new world looking for any surface caves. You can find a normal Woden chest with some money in it and usually a magic mirror. Keep on repeating this process until you find one. Good luck!!