Moved to S4 for better responses and flow of info.

 

There are a ton of them available on Ebay. You can find carbon fiber, metal, and plastic replacements. I haven't seen any sites selling anything other than OEM replacements however.

 

Is there anyone selling one that is just like the stock piece but has some texture to it (like the S2)?

 

Yes. Check eBay and Amazon. Lots of choices

 

Has anyone seen any carbon fibre replacement backs?

There are lots on eBay that claim to be carbon fibre, but even a cursory glance at the photographs makes it clear that this is just a slightly carbon weave pattern imprinted on plastic or PU leather, not actually carbon fibre at all.

 

I think a metal back with good texture would add some much needed heft to the phone. Too light by itself so I've been using a Spigen Neo Hybrid case.

But I worry that putting a third-party metal backing on the phone might interfere with the antennae since the phone wasn't designed to get an ideal signal while accounting for a metal case.