have a hack that have xray in it and let you select the blocks you want to see
If you want to do it legit, just explore caves.
I found 2 days in a row, 2 spawner = 4 spawners ( 2 zombie,2 skeleton)
Just by roaming around in caves with my friends.
Weepcraft is a good client that allows you to toggle which blocks you want to see - It's what I use for finding spawners.
Just download Forge and google for "Minecraft XRay Mod - Configurable" and click on the first MinecraftForum link. You should find all the info there.