hi guys im looking for some aftermarket rear coil springs for my audi a4 s line convertible 2006 2.0 tdi

can't seem to find anywhere that sells them apart from dealers 

Your S-line came from the factory with sports suspension as standard (as does my A4).

 

Why not look at changing all the springs using Eibach or another quality make?

 

Eibach do the Pro Kit which can be used with the standard shocks and is top quality kit!  It will lower your car by about 20mm and costs around £220.  In real terms it will only lower by about 10mm as your car is lower than a non Sport/S-line by about 10mm.