Infernal Bolts have the 2nd highest attack of all ammunition available for Crossbows in the game (only beaten by Spectral Bolts).
These bolts are not available in NPC's stores, making them a rarely used ammunition. The easiest way to get them is by providing a special material to a blacksmith, who will craft them. In the Summer Update 2012, the attack and level requirement were changed from 42 to 72 and level 70 to 110.
You can obtain 100 Infernal Bolts in The Ultimate Challenges Quest Warlord Level.