Yup, the best ones available are the factory third-revision magazines with the cast alloy body. Should be able to find em at pretty much any of the big sporting goods outfits.
As aa1911 posted, they're a stinker to load up to full 10 round capacity. I usually stop at 8. Easier on the thumbs that way.
You can get a 30-round magazine from Remington, but they're plastic and I haven't had very good luck with them feeding consistently after a little use. Kinda clumsy to use too since they stick out right about where the rifle balances so carrying it one-handed with one of them in the mag well is a pain.
As for other accesories, you're right that there isn't much available for the 597.
You can get the ProMag Archangel "tacti-cool" stock plus a few other synthetic or laminate stocks, scope/picatinny rail mounts, and a few big banana-clip style 30-rnd magazines. Volquartson makes a few drop-in action parts (Their Exact-Edge extractor is a worthwhile investment since the 597 does tend to beat up the factory extractor after several thousand rounds). That's about all that I've found.
I'm happy with my SS/laminate 597 so I haven't tricked it out with all sorts of doo-dads. It's pretty much stock on the outside with a little bit of trigger tuning on the inside.