Best price I've found on GB was mil/LE only, which I can no longer claim. $365-$375 is about the going rate from I can tell. I check Osage County, Top Gun Supply, and such as my regular P320 supply haunts.
That said, ACME in Seymour has also been good on mags, which are harder to find than I expected. Good price on them too ($36). I've not seen cal x-change kits in stock locally.