Over the last 2 weeks, I heard a loud squeaking noise. Not the common clonk (over short suspension drops) but moaning during longer compressions. Coming from the right rear.
Today, I looked under the car in that area. Looks like the last loop of the spring is broken.

A quick search at the usual parts stores did not bring up a replacement spring; rather everyone is overing the Eibach, H&R, Vogtland lowering springs (1.4 inch typically) for around $200+ for 4, Vogtland seems to have the best deals.

RockAuto has a variety of brands (Monroe, KYB, Quinton Hazell, Boge) for $40-$60/ea.
Can I just buy one?
What is the OEM manufacturer?
Or am I better of getting all 4 with a lowering kit? Car is NOT a performance car (otherwise decision would be easy), but lowering is an option (assuming it improves handling without too much ride penalty).

 

I have had Eibach on another car and H&R which is currently on my A4 and I have to say that they are both great springs that still retain decent ride quality. My advice is to replace them all and enjoy a lowered look.

If you don't lower the car and decide to stay "stock"r don't just replace one rear spring, you might as well do both.

 

Get a set of S4 springs used - you can probably find them with struts for $125/set. Fantastic upgrade over A4 suspension - it's what should've come with the car to begin with.

 

Springs and shocks should always be done in pairs. Look for a pair or set of used springs at a local yard, or Shokan if you just need replacement parts.

Otherwise, just shop Tirerack, ECS, or Achtuning for the best deal. You'll need shocks and springs to lower the car for each corner. Monroe is a total waste of money. Stick with KYB or Bilstein replacements and Eibach or H&R springs.

Achtuning offers great voitland packages too.

 

In a box, I have a set of stock springs from my wife's 2001 Audi A4 1.8TQ with about 20k miles. I have the shocks/struts as well. Let me know if you are interested in them.