My 229 uses the 228/29 13rd mags,,I've recently sent the pistol to sig and had the AEP and new sights installed
I have bought a 357/40 slide and 357 barrel to convert it ,,
I have not been able to find and 357/40 mags that will fit it ,,any help on what kind I need and where to find rhem ,,,Would like to have at least 3,,

None of the .40/.357 P229 mags, whether factory or aftermarket, will fit your P228/P229(9) old frame, because, as you found out, the mags are wider at the bottom than the magwell. You are going to have to use the P226 mag. There are no other ways.