If you feel adventurous (and can't find the actual bracket or don't want to buy it in bulk), you could modify the existing bracket to fit the SFF case. I've done this before and while it took a bit of effort it worked just fine.
If you want to do this, remove the existing bracket from the network card (to avoid damaging the card), measure where the bend in the bracket should be in order to fit the SFF case, use a good pair of pliers or a vise or clamp to carefully bend the bracket at that point. Then measure where the bent end of the bracket needs to be cut off in order to fit properly on the frame of the case and cut it at that point. Finally, use a Dremel or other grinding tool to cut the indentation on the bent part of the bracket to fit over the retaining screw hole. Reattach the bracket to the network card and install the card in the case.
I do highly recommend trying this out first (for a bit of trial and error practice) with the bracket from an old PCI card that you no longer want so that if you make a mistake you won't be ruining the bracket that you need for the Gigabit card.
