Thanks.A P6 (and I'm welcome to be corrected if I'm wrong) is just the police version of the P225. The hammer spur is different, but as far as I know, all parts interchange.
Numrich has a lot of the parts, should you need any.
https://www.gunpartscorp.com/gun-manufacturer/sig-sauer/auto-pistols-sig/225-sig
EFK makes barrels, as well.
Magazines are pretty much $40-50 German mags, or Pro-Mag...not a lot of choices there.
As far as interchangeability with newer Sigs...that I do not know.