Zhonya's is a decent option, you get the activate, armor, and some cdr. Also if you have AP scaling it may be almost as good as it would be on mages.
GA is a more generalist option that has both resistances.
Steraks gives less defensiveness with no resistances but also gives a bit of damage, so you can take it as a half measure if you think it will be good enough.
And then of course you can always just buy Randuin's for the hp/armor, it also helps you against any other auto attackers and can slow people that jump you to make kiting easier.
There are a lot of options, but in most circumstances now if you want to survive against an assassin you are going to have to pick up at least one defensive item. Honestly that does not seem too unreasonable to me but apparently I am in the minority.