Adding to what fawful14 said, there actually are people in the real world that drink blood as their main source of nourishment. They're sanguinarians. Where do they get their blood? People that willingfuly offer their blood: blood donors.
And guess what, "vampire slayers" also exist because of this. Don't worry, they don't actually kill "vampires", they're just manic christians that send them hate mail.