In order to find a nether fortress, first you need to go into nether.
Now open your coordinates menu and follow either the X or Z direction.
Here we are the following Z direction.
You have a better chance of finding a nether fortress if you follow one of these directions in a straight line.
After about eight hundred blocks, I found a nether fortress.
Nether fortresses have blaze spawners, you can kill them to get blaze rods and blaze powder.
They also spawn wither skeletons. You can also find loot chests in the Nether fortress.
Open it up to check what is inside it. Add these things into inventory. These chests are the best way to obtain horse armour.