1) If you haven't done the "Destroying the Dawnguard" quest yet, get it active. If your illusion skill is good enough, go into Fort Dawnguard, but don't kill the leaders as you are supposed to. Calm Sorine and Gunmar and talk to them,then you can buy all the bolts and even crossbows they have. Sadly, though, you cannot to Sorine's Ancient Technology Quests..... Oh, well, you still get bolts!!!
2) Remember the infinite arrows that we could get from the guard in Solitude? Same idea, you just need a little better pickpocket skill. Pickpocket the bow and the arrows off the guard (or anyone else shooting arrows) and give him a Crossbow and a Bolt. He will start shooting bolts, now. Awesome right? Now, if you combine this with dwarven bolts, which you can get off Dwarven Spheres, then you haven easy way to get them, too!
That is all that I've found, but has anyone else found anything? Thanks and your welcome!