I read somewhere that crossbows and their bolts can be "smithed" using the right materials, however I've recently started Dawnguard and can see no option for these anywhere in the Smithing menu. Is there a special perk I need to be able to smith them or was it a mod that allows you to do it?

You need the schematics for it and asking Sorine Jurard how to create it, and can only be made at the forge in Fort Dawnguard. So, you have to progress a certain distance before you can do it.

Ah. So if I join the Vampire Lords, no "infinite" bolts for me

If you join the Vampires there's a few things you cannot get including improved crossbows, bolts, and an entire location that has 5 unique books and a unique weapon.

since we're in the spoilers section, were is this location with the unique weapon?

Schematics are found at random locations given in radiant quests, first it the improved crossbow, then the bolts then the dwarven ones last.

Have you ever found the 4th volume of Excavating Discovering Runvaald? I am not sure it is actually in the game.

Edit: I just looked at the uesp wiki. The other three volumes are noted as being located in Runvaald. The fourth volume does not have this notation. So maybe it is just in the database.

http://www.uesp.net/wiki/Skyrim:Ruunvald is a location that only materialises if you join the Dawnguard faction and do a quest, and there's a unique staff on the boss NPC.

Because my main game has followed the Vampire faction, I gave them to myself with the console. I collect the books, but in this case it was never going to be possible for me to collect them so...

The notes that show up in the chests with the schematics for improving the crossbow and bolts probably never occur in a Vampire faction game either. I still haven't checked in game if the dwarven caches show up when you're not Dawnguard, but they probably don't.
You can loot some enchanted bolts in the game as a Vampire, and one of the things I'll do when I next play is check if they respawn. I've recently discovered that a lot of stuff in Skyrim respawns, including 'unique' journals and notes, and even the knife and fork weapons found in
Edit: In my game the 3 types of Exploding Steel Bolts you can pick up from the table in Dead Drop Falls have all respawned, so theoretically you could continue to get these bolts if you wait out the respawn. The only bolts I don't think you can get are the Dwarven Exploding ones (the plain ones can be looted).

ah, the junk charming staff. gotcha. was soo hoping you ment a sword or something. kinda grumpy at the distinct lack of dawngaurd swords. Thanks just the same.