Replacing Sora crankarm so I accidentally screwed the wrong side pedal into my left hand crankarm on my Giant Defy 3 2014, and I have been looking to buy a replacement part myself and get a mechanic to install it. 

I am wondering what would be compatible with the original Sora groupset components? I haven't been able to find just a crankarm, so I am looking to buy a crankset.

I can get a Shimano Sora Double 3550 9 Speed Chainset for 40GBP. 

Would other 9 speed cranksets work?

What other left hand crankarms would be compatible?

Thanks.
If you're replacing the whole crankset and BB, then any 9-speed double crankset should work. The chain width for 9-speed systems is the same for all makes (not the case for 10 or 11 speed).

But before you do that, I'd look around and see if you can replace just the left  arm. Any crank designed for the Shimano "outboard bearing" BB should work (those are the ones that clamp onto the BB spindle with two pinch bolts). Just make sure the crank arm is the same length.