80% P228 frames are available from [url]www.ktordnance.com[/url] and [url]www.dlask.com[/url]. Dlask's are cast steel frames (I think), and KT's are milled from solid steel or if you want, from aluminum.
DLASK also has part kits (or at least HAD them last time I looked). There is also a part kit (less the barrel) currently listed on ebay.
KT frames are milled in (2) halves and he has them brazed together. He may we willing to sell a frame that hasn't been brazed. His email is on his site, so you can ask him.
as for the DLASK part kits, they are not drop in parts, and many are out of spec, but they are significantly cheaper than SIG OEM parts. I also suggest you check out [url]www.roderuscustom.tzo.com[/url]. They have a forum for Sig P228 builds that will provide most of the information and details you will need.