For SIGs I only recommend SIG brand mags or Mec-Gar mags. Pretty much everything else doesn't work well. Some new 226 9mms are coming with Check-Mate makes which are proving to work pretty well.
For spare parts I only recommend is a recoil spring.
SIG recommends replacing the recoil spring every 5K rounds or when the muzzle end of the spring starts coming apart.
If replacing at 5K rounds I also recommend replacing the mag springs.
If you have an older folded metal carbon steel slide I recommend changing the decocker spring and breach block pins at 10K rounds.