Where is the best place to get a replacement recoil spring and guide rod for a SIG P228 9mm?  I am looking for a factory replacement.

TGS 
Pretty much for most Sig parts.

Yep. OEM Sig braided springs are the best. Avoid the Wolff flat springs. 5000 rounds is the recommended replacement threshold, but if you are shooting mostly low power range ammo, you can stretch that a bit if you care to.

Best option is to get the parts kit (about $25) and replace all of the parts that are recommended as maintenance items