Nether Fortresses tend to generate in strips along the Z axis of The Nether, so if you find one fortress, you can find more easily by then travelling along the Z axis. If you are after wart and rods, don't wait too long, go as soon as you can, or you will need to walk thousands of blocks to find an un-looted fortress.
Nether Fortresses contain 1 or more Blaze Spawners. Even if a Nether Fortress has been found/raided the Blaze Spawners are often left intact. You may expect to find blazes in most fortresses even if they have been raided.
Blaze farms are not easy to make but can yield great results. Details about how to make a blaze farm can be found here BlazeFarm
Some players will keep Blaze Rods on them at all times in case their base is raided. You may be able to kill them for their treasure, but remember that if they have Blaze Rods they may also have potions.
Some players will be willing to trade Blaze Rods for diamonds. Expect Blaze Rods to be expensive.
Occassionally players will give out Blaze Rods randomly.
Farming Blaze Rods is no simple matter. The Blaze mob farm must be built in the nether The Nether. It will, by necessity be in the open and therefore subject to being griefed.