I don't know where Baer gets his slides and frames, but a company called Jerico supplied slides and frames to both Kimber and Wilson (althought to different specs). Kimber bought out Jerico, so I guess you can say that Kimber now supplies slides and frames to Wilson. However, note that they are not made to the same specs as Kimbers own slides and frames.
Also, I believe Jerico got its base forgings from Smith and Wesson. Although this really doesn't mean much, I guess you could say, in a roundabout way, S&W makes Wilson slides and frames.
As you can see, it can all be very misleading.
Oh yeah, the only company that uses a cast frame, as far as I know, is Para Ordnance. However, I would not worry about this, people make a bigger deal out of this than it is. There was a time when castings were definitely of dubious quality compared to forgings, but nowdays they are very close. I know people that compete with Paras who have put over 100,000 rounds on a cast Para frame with no problems.