My version of vampires were created by powerful mages during their experiments-they sought immortality, so humanity would never end up without someone to save them from the eldritch horrors or creatures of chaos that would otherwise destroy them. However, these vampires have the following problems:
Blood Dependence-They need blood to sustain their life; a minimum of a liter for every two months. The average person contains 4 to 6 liters in their body (men generally have more than women), and someone can die from losing 2 1/2 to 4 liters of blood (exsanguination). Now, a liter is 2.1 pints, and it takes four to eight weeks to regain just one liter of blood (plasma, platelets and red blood cells alike). Vampires just need the blood to sustain their life force; for nutrients and calories, they can ingest regular food for that.
Blood Dependence (Continued)-Vampires get weaker over time, as the life force (AKA essence, spiritual energy) inside ingested blood is depleted. For this and other reasons (seriously, could you perform your best when giving your body only the minimum amount of calories and nutrients it needs?), vampires really need regular blood meals (at the end of every month? I'm open to change).
Blood Hanger-Going off of 2, did you know a 1% drop in your bodily water reserves may cause a proportional drop in cognitive function? Combine that and hanger (hunger-driven anger), and you know exactly why a vampire who's gone without blood for a while is a potential danger. The more blood deprived a vampire is, the more primal and ferocious they become.
Imprinting-The essence contained in the blood causes vampires to form a very close connection to those they've fed on for twelve months or more: they can sense their presence (and by extension, their location) and emotional state. This is called 'Imprinting,' and it makes it perfectly likely that a vampire can hunt down (and quite possibly stalk) a former donor.
Picky Bloodsuckers-Vampires only Imprint on certain people; they just can't take blood from anyone, unless they're dying from blood deprivation. They seem to sense the traits of the person who the blood came from and judge it by that; in simpler terms, a vampire's choice of blood says a lot about the kind of people they like to be around (and them by extension). The exception is vampires that can't (or won't) Imprint; they don't really care who their blood comes from.
Even Pickier Bloodsuckers-For some reason, vampires tend to seek donors that can also be partners; male vampires have an innate drive to seek out female blood donors around their age (no more than 4 years difference; see below) and vice versa for female vampires. The difference in blood drive is dramatic; most male vampires would never even think about taking blood from a male unless they had to (and once again, it's vice versa).
The Good News:
Vampiric Healing-Vampires have anti-coagulant agents in their saliva, but they also have healing agents in special glands they can pump into a donor to heal the damage (of the fangs puncturing the skin and blood vessels) and replenish blood cells 4x as fast as normal. This prevents scarring (AKA fang marks) and helps the vampire get more frequent blood meals without hurting the donor.
Imprinting-And even just donating blood to a vampire once-creates neural links in a vampire's mind, making them much less likely to attack, steal from, or dominate you (sadly, 3% of vampires have mind-control abilities) and more likely to help you. Generally, a vampire will do anything to help their Imprinted. However (see 4 above) this also causes a vampire to very quickly develop relationships with people who give them blood. If an older individual acts as their donor, the vampire will soon see them as a paternal figure. If a member of the same sex, as old as them or younger, acts as their donor, the vampire will soon see them as a sibling (brings a whole new meaning to 'blood brother,' now doesn't it?). If a member of the opposite gender, around their age (no more than 4 years difference) gives a vampire blood, they will likely develop romantic feelings for them.
Imprinted Food-No, it's not what you think. After a vampire has Imprinted, food made or grown by that Imprinted are as good as blood for them (well, about; the caloric value of the food determines the amount of BV, or Blood Value, a food has). So food made by an Imprinted can be better than blood, and it's a whole lot easier on logistics. However, vampires often take blood now and then, out of habit or, y'know, the fact they're vampires.
The vampires are legal citizens, the governments knows they exist (regular politicians are kinda-sorta bloodsuckers anyway, and since these vampires are better allies (and taxpayers) than enemies...) and is willing to turn a blind eye as long as they don't start enslaving people, causing crimes, or draining them dry. However, in order to survive, the vampires need blood (and it can't come from blood banks; vampires don't usually need medical aid, so the government has reserved that for the regular people who end up hospitalized and lacking blood) from donors.
Even worse, due to all the anti-vampires crazies (and those who suffer from a fear of all that sucks blood, and those who would not take the news of vampires being real well), the vampires have to keep their existence a well-kept secret or deal with an awful lot of prejudice and discrimination.
My question is, simply: How Can Vampires Get Enough Donors To Keep Themselves Alive?
Specifically, what would be the best method to obtain a donor? I feel like a dating website could work, but I'm not sure how effective that would be. The best method won't just attract crazy people, will incentivize nondisclosure (of the vampire's existence), and will get people to willingly give blood to the vampires.
EDIT: As pointed out by DWKraus, a vampire should only need one donor, due to math (a full vampire taking frequent small meals should never leave a donor more than a pint down) and modern medications that can stimulate blood production.
If more details are needed, or if there are problems with the OP, please let me know. As always, I appreciate your input and feedback. Thank you!
EDIT: In the end, I found DWKraus's answer the most helpful, but each of you had an important contribution (except Ash...sorry, but I just can't go that route).