Spawners Vary, there is no spawner level. Some are on the surface but most I would say are from y:55-y:20
You won't find too many below 20 and if you do it's uncommon.


From MCWiki:

Spoiler

Occurrence
A monster spawner with a skeleton inside.
Spawners can occur in these places, spawning mobs chosen randomly when generated:
Dungeons
Spawns Zombies (50% chance), Skeletons (25%), or Spiders (25%). One per dungeon. Sometimes a Spider Spawner will also spawn a Spider Jockey.
Abandoned Mine Shafts
Spawns Cave Spiders. Located in horizontal tunnels, heavily surrounded by Cobwebs.
Strongholds
Spawns Silverfish. Located in the End Portal room. One per stronghold.
Nether Fortresses
Spawns Blazes. Located on Nether fortress balconies with stairs leading up to them.
Naturally occurring spawners will not spawn passive animals, Creepers, or Endermen. However, if a map editor or mod is used to create a spawner for any mob (although correct conditions must be upheld for the spawners to actually spawn a mob), it will function as expected (so it will even spawn Ender dragons)
There is no way to obtain a spawner in your inventory in Survival mode without turning on cheats, using mods with an inventory editor program, or using the "/give" operator command in a multiplayer server. It does not appear in the Creative mode inventory, but can be obtained using the pick block feature (before 1.2.5).
If such a spawner is placed, or if a spawner is missing its tile entity data, it will spawn the same entity, but will have a pig inside.