Armoured zombies are great, like somebody mentioned above. Also, the desert bandit camp is a great place to train attack with minimal attention. You can AFK here continuously while you chat on MSN/browse forums/watch Youtube videos etc. and the cost is minimal (you pretty much only need to spend money on prayer potions). You can also train with Dharok's at the monkey guards (like somebody mentioned up above). I have a wicked spot to train with Dharok's that has fairly good loot that I can show you if you PM me or message me ingame. It is where I got 96-99 strength on my barrows pure
Also, if you want to train with me ingame that would be cool! My char is currently 90 99 75 and I am planning on training him up to 99 attack aswell. Let me know if you are interested!