I would probably buy a spring kit from Wolf. Something similar to this, after confirming the correct part no. For your specific Sig.
Wolff Hammer Spring Pack Sig Sauer P225 P226 P228 P229 P220 Above SN
I assume this will be a "range" gun. If not, be aware that reducing spring weight can result in light primer strikes and therefore create potential reliability issues.
If you haven't detail stripped the 229 before, Top Gun Supply sells a very good video done by Chris Orndorff, and a Sig Armorers tool makes the job a little easier too. It has the proper hollow ground screwdriver tip for the grip panels, a tip for replacing the mainspring (on some models) and a tip to easily remove/ replace the extractor and also to remove/replace the sear spring.
Hope this helps.