The boomslang thing where you go into snape's office and hit a on the pic..

Yes,
Miltank can be caught in the new section of the safrie zone (sorry for spelling). It is very rare so it mite take a while to catch. Note: To to this you need to beat the elite 4

Yes...

If you want to find the Boomslang Skin, first go outside Hogwarts. When you are outside, walk south in a straight line following the dark brown ground. Stop walking when you reach the 1st four trees that are place equidistant to each other(to find these trees, you must walk left and check if you are aline with the entrance gate of Hagrid's Hut). If you are now on the trees that I have said, put yourself in the center of the four trees then walk south (12 steps). Walk south a little bit (12 steps) and walk to your right and you must see a plant that is something similar to a tall grass that possesses the color yellow-green or something related to green. Go infront of it and press A and you will receive a Boomslang Skin(you can collect Boomslang Skins as many as you want but to do that you must hit Start+B+A and receive another Boomslang Skin).Good luck!!! Happy Hunting!!! ^-^

That answer does not work if you are looking for it to heal Neville. If that wasn't the first ingredient you obtained then you can't get it, meaning you cannot get past it. It was a flawed idea for the makers to assume that everyone would just assume to look there first for the boomslang skin. I made the same mistake the first time I got there. It really ticked me off because I went searching all over the place for that thing.