So instinctively I started searching for Nokia original cases for the phone.
Amazon has a few, though they are not original Nokia silicone cases that I would prefer, but they are cheap compared to what At&t is offering, they cost between $5 and $15 for the ones I saw.
At&t offers silicone casings and bumpers too, with the option to choose whatever color of your choice (black, cyan pink green), but they are pricey at $20
you can see Amazon's range of Lumia 900 accessories HERE.
At&t also has their own stash which you can check out HERE
unfortunately I cannot find a white casing anywhere, hopefully it should eventually come out sooner than later!