For that type of spring ("mini torsion spring" or "V Spring"), I'd just got to a local ACE or True Value or similar large hardware store (vs home improvement store). Bring the original and I expect you'll leave with a replacement pretty cheap.
They can also be ordered online from any of the usual suspects or bought in kits with a variety on eBay or Amazon.
You can often just eye ball it. But, to order an exact replacement online, I think you need to know the direction of the wind (left hand wrap or right hand wrap), inside and outside diameter of the center coil, the number of times the coil wraps around, the gauge or diameter of the wire, the angle between the legs, and the length of the legs.
But, again, I bet you can walk into a big, true hardware store with the broken part and walk out with a replacement for less than a $1 CAN.
Good luck!