Anyone know where I could track down a OEM rear sight for a Ruger GSR 308win? Also where's the best place to order mags?

Have you called Ruger CS?  They would probably be the best source for the OEM part, and may send you one for free.

As for mags, back when Ruger was selling the waffle looking mags I bought a few of those, but I also have some Magpul AICS style mags for my scout; all are shorter, lighter, and easier to load than the metal mag in came with.

If you're asking about metal mags, I have no idea and can't help you there.

I'll reach out to Ruger and see if I get lucky, appreciate the help on mags.

They might ask you for the rifle serial number.

Looks like you will have to call customer service. Shopruger.com does not have that part that fits onto the rear scope ring base. They still have the polymer 308 magazines in 3 capacities but the steel magazine like originally came with the rifle is no longer on that website. The Magpul AI style magazines also fit.

ETA:  Talked to one of the Ruger retirees I know. He says customer service is your best bet. If he was still working he could probably have hooked you up.

XS Sights makes a rear peep aperture which, IIRC, can be drilled and tapped to accept screw-in apertures of varying internal apertures/diameters.  This XS rear peep is threaded differently than the OEM Ruger peep and requires its' own threaded plate which substitutes for the similar OEM rear sight plate.  By itself, the XS rear peep, after drilling and tapping, makes a perfect "Ghost ring" rear peep.  With apertures installed, accuracy in good light is enhanced.

This is not listed on XS Site, so you will need to contact them and work things out.  The Peep and its' plate can be seen installed on XS Ruger GSR rail in the preceding link.  Here is a link to the
Internally Threaded Rear Aperture    Just click the button for "threaded aperture".  You will need to also order the plate into which it screws.