If you go to Whistling Mine at night (just outside of Winterhold). You can find a couple guys that are sleeping on a bedroll. Should be easy to feed off of one of them; as long as they are sleeping and no one else is wandering around.
Personally installed a mod that allows me to collect race-specific blood as ingredients that can be used in alchemy, but I myself added a few recepies that allows me to take two units of race specific blood and mix them into generic potions of blood that have the same effects as feeding on a living person, allowing me to slaughter people, harvest their blood and drink it at will >:3
I can also mix blood potions into alcohol, (blood mead, blood wine) that you can also drink to reduce thirst and feel classy. MODS FOR LIFE!!!