[davis328xi]

Hi,
I have an '07 328xi 2D coupe, and am getting a bit frustrated with reoccurring topical rust on my brake discs. It's mostly on the rear wheels, but am noticing it some on the front as well. I've had the detailers wire brush it off before, but that's just a temporary fix, and would like to replace all four brake discs with a rustproof material if possible.
Does anyone know what the brake disc size is I should be looking for (have stock 17" rims)? What material should I look into (stainless steel?)? And also are there any websites you'd recommend? I know I'll need my (st)ealership to install them, but would like to buy them cheaper online if possible.

Thanks for all your help!

LT