where can i get blue dragon vambraces?

Can only be bought from other players who have them or who are able to craft them themselves.

Or craft them yourself ofcourse. If you have the crafting level you can go to the tanner on the ranging or crafting guild to craft you a hide, then use a needle with thread on it

Craft them yourself if your crafting level is high enough or buy them from players. Go to Varrock bank and there are usually players there crafting d hides.