There are no real alternatives to OEM on the basis of changing a sole spring due to their size.
Best bet is Eibach if youre pushing the boat out and having all four springs done. Failing that you could try a breakers for a sole rear spring whilst informing them your car has m sport suspension......
I knew the spring had gone before I had purchased it. The previous owner who is a friend agreed to pay for it to be done but the local dealer who initially said it would be done last week and then said this week and now they are saying in 2 weeks.
I'd happily do it myself if I could get hold of the part. Looks like I'll just have to wait
Not sure if its any help but the si springs from both the 2.5 and 3.0 are identical on the rears. I established this whilst doing homework into the failed rear OSR spring on mine.
Rear springs are easy to change due to only requiring one bolt to be undone.
I know that there are none in the UK at all. Might be a stock lowering activity for end of financial year or a reaction to all the forum members checking their springs and requiring units. Either way, I have been unable to locate one. Booked in for all four Eibachs in just over a weeks time.
Spring table is hidden in the ETK - under 'Parts Search - Specific - Spring Table'
for my car which is an 04 3.0 SE with MSport (as I believe they all are?) Part numbers are:
Front: 31336760622 or 3
Rear: 33536761925 or 6
... But it does ask you what options you have fitted (such as electric seats, SMG, Stereo system, Headlight cleaning)... some of those options seem to change the part numbers slightly :?