I currently use these items from anker. It is a quick charge 2.0 enabled charger, which you don't have on an m8 AFAIK. For a cheaper option that doesn't have the quick charge 2.0 option, see here. You even get two adapters for $10 with this item, as opposed to the $16 for 1 quick charge-enabled adapter.
I just bought this anker charger and it works great. It sounds like you are just looking for a 2 amp charger which is what the phone came with.