I wish to do pickled onions but am unable to get Brown vinegar only Apple Cider Vinegar
Pretty much any type of vinegar works. 

update:
Responding to comment below, I guess I would stay away from distilled white vinegar and and any "heavy" types such as balsamico, jejiang, shanxi and so on. Apple cider vinegar, white or red wine vinegar, sherry vinegar and so on would all work well.