After Dawnguard is installed I know Dragonbone weapons can be crafted, but will they be found in loot and at vendors too?

I have never found a dragonbone weapon as loot, not counting the 3 keepers you fight in the soul cairn.

They're WAY more rare than Daedric weapons, which are pretty rare themselves.

Havn't you got the DLC yet? Half the fun is letting your character find out these things. The reason I ask is that you previously stated that you own both a PS3 and an xbox, just wondering why you are holding back on the DLC. You should get them, they're all really good.