I have a PH friend who just purchased a push feed Winchester .458 that came with a scope. Unfortunately, the previous owner had removed the factory sights. Does anyone know where I might be able to locate some of these? Thank you.

I would try Brownell's.

The Williams front sight screws right on and is the same as what came on the rifle. I've never checked but the rear should be available there too. I'm not fond of the rear site that came on these as it is folding and I wish I could find a standing site for it.

Try this firm:
http://www.newenglandcustomgun.com/

On mine I have substituted the front blade and hood with a Williams Firesight rifle bead and their hood with the hole on top to let in light to illuminate the fiber optic bead. It works great.

http://www.williamsgunsight.com/gunsights/rifle.htm