You can get it from any site that you can download mods from, such as tesnexus.com.
It works like any other mod, you download it, then put some of the files in your data folder of the oblivion folder and then activate the mod. there should be instructions in the readme. I don't remember if that mod requires obse or not, you might want to check that beforehand.
After you put them in you Oblivion/Data folder, open the oblivion launcher, and go to data files. Find the name of the mod and check it to activate it. Now, when you start your save/new game, you will get a spell called mount selection. Cast it and choose the mount you want. You will then get a spell called Command "Mount you chose." Cast it, and select summon. Wa-la, enjoy your dragon.