You don't have to replace the springs in pairs - but - you need to match the new one to the length, internal diameter (1.9" or 2.25" usually) and rate (how much force it requires to compress the spring) of the old, broken one. However, if one is broken, could the other be on the way out too? A pair of springs is about £50, so could be worth changing both at the same time.
The dampers are a completely separate unit, so unless these have their own issues, they don't need replacing.
The springs are generic items, so don't have to be purchased from Westfield.
Faulkner produce decent quality springs at the price I mentioned, and have a wide range of sizes and spring rates available. They can be bought from the usual suspects - JJC, Demon Tweeks, Merlin Motorsport, eBay, etc.
Hope that helps