Springs are always matched, there is no one stock spring or Eibach spring. Being aftermarket though, the Eibach options are simplified, you just choose the engine (318i, 320i, 325i etc.) and the body type.

S54 is heavier than the M54 so an OEM+ setup like in this guide might not be for you. In such a case I would probably get M3 coilovers from Sachs or Eibach.