I’m currently a lvl 28 nightblade, and I have yet to run into a sword better than my lvl 16 ones. Where should i farm for betters ones? I’m only on normal, should i farm or keep going?
Keep going. I had to use a level 50 mace until level 85 because I couldn’t find anything better.
If you switch to Veteran then you get more hero spawns which gives more chance of good loot 
Play on veteran its much better for drops and double hero spawns are loot fests.
For epic (and later legendary) swords anywhere works - most will come from heros and I can’t think of any boss that carries non-MI swords frequently.
For MI swords there are good ones in Dermaptern hive* (and you get lots of chances at one per run) and if you side with Kymon’s chosen there is a skeletal boss who drops a really nice MI sword for cold/pierce hybrids.
*I assume sense you mentioned swords specifically that you want pierce damage centric swords