Plated Tabi Ninja Steelcap Boot
Tabi is a fine choice when behind! I don't like every going this item, but if I die early to Akshan I might consider it.
Doran's ring
I haven't tried this yet, but I imagine it's the same deal as Vayne, where you can just max range Q minions (or Q enemy champ for HP) and not run out of mana, playing super safe early. Counter-intuitive, I know.
Iceborn Gauntlet
Iceborn and Dead Man's are both fine options, but when I'm ahead I like to go Heartsteel. Iceborn will make it even harder for him to escape though, so if it's good elsewhere on the map it might be worth building!