Getting Nether Warts and Blaze Rods
Nether Warts and Blaze rods can be found in only a few places, and are needed to make all potions.
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.
Don't expect to find any Nether Wart in fortresses that have obviously been raided. Nether Wart will probably still exist at the far reaches of The Nether, far outside of the HardcoreSMP Overworld map range.
If you are lucky you may stumble across someone else's Nether Wart farm. Players regularly hide these at the top and bottom of the map, but it could be a long search, and you need to be wary as someone may be coming back shortly, or lying in wait for you.
Some players will keep Nether Wart 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 Nether Wart they may also have potions.
Some players will be willing to trade Nether Wart for diamonds. Expect your first wart to be expensive, but once you have some their value decreases significantly as more can be grown.
Other players will give out Nether Wart randomly.
Farming Nether Wart is a simple matter of planting them in The Nether or OverWorld in soul sand.
Nether Wart has three stages of growth, and you can uproot it safely at any stage, however if it is uprooted during stages 1 or 2, it will only return the single Nether Wart to you. During the third stage wart will return more. According to the Minecraft Wiki, a tool with fortune should increase the yield.
Like all plants, they will only grow when players are near, so you must hang around your farm.
Leaving Nether Wart unattended is not usually recommended, as it are one of the most valuable resources in the game, and anyone finding your supply will almost certainly steal it.
A clever tactic is to hide a Nether Wart farm near to the Netherspawn at 0, 0, where passing players will keep your farm loaded and growing well.
There is a common misconception that you need to place lava near your wart, this is untrue.
You can harvest fully grown wart with a tool enchanted with fortune to get a higher yield.