Not sure if this is true for all worlds. It was easy to find for me. Just drop down any of the pits in the corruption and there is probably a demon altar at the bottom (or you can at least see one). Granted you need a lucky horseshoe if you just expect to fall the whole way. Found like 5 altars this way.
You need a pickaxe which you get from materials from killing bosses, or use the powder that you get from the druid that comes after you kill bosses, either way to get to that one you need the nightmare pick axe or higher.
YOu could just look for another one that is not near corruption. There's a few but yeah good luck finding it =(
You should read your Bible, sirs. You'll find all types of weird stuff in there. Like, did you know Jesus was a Jew? -Jay
Most of the altars will generated in brown dirt/rock layer of underground (rarely found in grey dirt/rock layer if corruption chasm reach that deeper area). If you dig around that area, you will certainly find one.
EL ELOHIM ELOHO ELOH M SEBAOTH ELION EIECH ADIER EIECH ADONAI JAN SADAI TETRAGRAMMATON SADAI AGIOS O THEOS ISCHIROS ATHANATOS AGLA AMEN - from SMT1 Opening