Sulfuron Hammer

Sulfuron Ingot are the hardest items to obtain to craft just this hammer. They have about a 22% drop rate from Golemagg so it'll be a while before you can farm it unless you go more than once a week using alts or another method.

Blood of the Mountain can be easy/annoying depending on how you look at it. If you have patience just keep mining Dark Iron Deposits in Burning Steppes and you'll eventually get enough.

Essence of Fire are easy, if you farm the other mats you should just have enough when you have the harder mats such as the Sulfuron Ingot and Blood of the Mountain, if not AH is a good friend. If you really want to just get this then head to Un'goro Crater and farm the fire elementals at Fire Plumb Ridge (center of the map).

Lava Core/Fiery Core may be a pain for some. MC trash can be cleared with a few people so if you can go in with friends and clear all the trash you should get enough or just short in one run. If not clear the trash 2-3 times and you'll definitely have enough.

Arcanite Bar are easy, just farm a bunch of thorium veins. You should get some Arcane Crystals from the veins and if you don't have enough they should be cheap on your AH.

So overall everything is moderately easy except Sulfuron Ingot being the hardest, followed by Lava Core/Fiery Core, followed by Blood of the Mountain then the rest. Assuming you going for the achievement Sulfuras, Hand of Ragnaros which most are, the Eye of Sulfuras is the hardest without a doubt the hardest item to obtain. It drops from Ragnaros and has a 3-5% drop rate.

Good luck if your going for the hammer/achievement :D

So overall everything is moderately easy except Sulfuron Ingot being the hardest, followed by Lava Core/Fiery Core, followed by Blood of the Mountain then the rest. Assuming you going for the achievement Sulfuras, Hand of Ragnaros which most are, the Eye of Sulfuras is the hardest without a doubt the hardest item to obtain. It drops from Ragnaros and has a 3-5% drop rate.

I started collecting the mats when I got the Eye. The Sulfuron Ingots are definately hard to find, and the Blood of the Mountain is also very rare.
The Lava and Fiery Cores are actually quite easy to get by farming the lava packs.

Also for the Arcanite don't forget that there's no cooldown on the transmutation anymore ! (I made them one by one at first -_-' )

Should be noted that post Cataclysm there are no Dark Iron Deposits in Searing Gorge / Burning Stepps anymore, they only exist inside BRD and MC, thus making the Blood of the mountain way way harder to get.