The short answer is I don't know of anyone commercially that has a stock of genuine USGI parts for sale. I see you already have a WTB ad in the Parts & Bits forum...
Another good source is to keep an eye on the tool boxes at gunshows, you never know when you will spot a few parts sitting about. Usually because someone has "upgraded" or changed out parts over the years, as in the one you already have. Canadian gunshows are hit & miss for this. The larger US gunshows will usually have someone who has USGI 1911A1 parts but expect to pay dearly for them.
The hardest parts to find for a Colt 1911A1 refurb are an original wide spur hammer & milled trigger [for an early WWII model] +/or original barrel that is in decent shape. The latter flat sided hammers & stamped triggers are not too difficult to find.
I did my own "resto" on an early WWII Colt 1911A1, that started out as a bare frame & some misc parts in a brown bag. I had been using it for a platform for a Colt .22 unit for many years before trying to resto it back into something that resembled a USGI 1911A1. However, it took quite awhile to get the parts for the project [including an era correct complete slide]:
NAA.