Sunstone is only found in lava pools. Because of this, it is one of the most dangerous ores to mine. And, not only is it dangerous, it is incredibly rare: the maximum size of a vein is 4 blocks.
In the Nether, it would be safer to use Stone to build around the ore, because ghasts are unable to destroy Stone blocks with their fireballs, meaning the only real threat is being knocked back into the lava. Fire resistance potions and spells are recommended.
Another way to mine sunstone is to make a projectile "create water" spell as create water works in the Nether.