I want to get as much Demonite ore as possible to make Shadow armor and to sell the leftover ore, but there is no way I could get enough from mining.What should I do?
The best way to get demonite ore is to either farm the EOW (eater of worlds) or EOC (eye of Cthulhu). If you want crimtane ore which is basically the same. Farm the BOC (brain of Cthulhu).
Have 1 world with the stuff you need to kill the eye of Cthulhu lots of times, then go make as many worlds as you need. It will take time to wait but it will spawn then kill it.
Definitely the best way is defeating the eater of worlds. He drops a ton of shadow scales and demonite ore. He even drops some when you've done some but not all damage to him. You can also rarely get armor similar to demonite armor from corruption enemies. If you didn't know how to spawn him, here's a link to the boss: