I've been fiddling with Karma builds, and I've found these item orders and things work best. First is for sustain, second is for AP-rushing for if you want to fight. Please note that both work out fine for support laning.
I've never gone middle as Karma yet, so I can't say if either are good for solo laning. But I'm guessing the second one is better.

First build:
Sapphire Crystal so you can use your moves more often at the start, as if you're duel laning you'll probably want to spam skills first off. Move up to Catalyst the Protector to Banshee's Veil for more protection and lane sustain. Skip this if your lane is all AD. Save it for later.
Ruby Crystal so you don't die without your boots, Catalyst the Protector again and then Rod of Ages so you do decent heals and damage. This'll also help you later in the game.
Mobility Boots is a great item by now, as you'll probably have to support other lanes by this point. Running between lanes is important. You could also get Mercury Treads if there is a lot of CC in the enemy team.
Glacial Shroud for that CDR and then Frozen Heart to get tanky and basically impossible to kill unless you get piled on.
Aegis of the Legion will help out your team... I'm guessing there is a better item than this, but it still seems like a cheap way to help your team.
Rabadon's Deathcap because your heal will do nothing at this point without it.
Running Quints so you can keep up and run away.

Second build:
This isn't really any different from the previous build, it's just in a different order.
Ruby Crystal is good for when you don't have a partner, because it's harder to use your W to escape. Catalyst the Protector and then Rod of Ages so you can nuke right away. Banshee's Veil to block attacks so you can be more aggressive.
Sorcerer's Shoes to help you punish champions.
Glacial Shroud for CDR and then Frozen Heart so the enemy can't fight back.
Blasting Wand as soon as possible so you can nuke earlier, then Rabadon's Deathcap for AP. <3
Aegis of the Legion last, as you are still a support, though AP nukish.
Exp. Quints so you level up nice and fast.

It's just what I use. If you think there's something better to use than Aegis of the Legion, I'll gladly take note.