Where do I find a crimson Altar?
Solution 1:
You will find most of the altars in the corruption or crimson area. Usually, there are caverns or chasms on the surface that will lead to areas with many altars and shadow orbs or crimson hearts (depending on whether your world is corruption or crimson). They are probably about 100-150 blocks down.
You can also find random altars in other biomes besides corruption or crimson, but they not very common. All the altars will all be at roughly the same depth, and aren't that far past the surface.