I tried to find one, but i failed to succeed, because my offering was . I am thinking to buy it
Keep trying. Go to zones like Rivenspire, Auridon, Shadowfen, etc... I typically like these areas because people seem to be nicer here. If you go to the big zones like Deshaan, Stormhaven, Reapers March, Craglorn, etc.. I tend to find people are just busier and out for themselves, where the other ares seem to be populated by more helpful people. Maybe it's all strictly my experience, but I think you'd have better luck with people there.
Go into Zone Chat, and ask for a Vampire Bite. You might have to pay, but more than likely someone will bite you for free. You'll just have to meet them at the altar (Google is your friend here) where they'll be eligible to bite you.
Note, this may take several weeks as the spawns occur at night, are random in appearing and type, and may get killed by others before you get there. You'll be running through the 10 locations every night phase untill you get lucky. For reference, vampire spawn is 3 vampires feasting on a corspe. If you pull a werewolf spawn, it will be 2 grey werewolves. Let them hit you until you get a new status effect, then run and leave them for others. You'll then need to travel to a wayshrine for the main town in your Alliance's 5th zone. Quest npc will be standing in front of wayshrine.
I always had luck finding blood fiends in Reaper's March, especially around Willowgrove shrine. My three vampire toons were turned this way, and it didn't take a long time (though having a bite from another player is much faster). Back in the day it was still novel enough that people used to call out the appearance of blood fiends in the zone chat. I think the chances of finding them increase when there's a new moon, but I've certainly seen them in other instances as well. I'm not sure if you have to wear armour or not when you fight them (I didn't ), but definitely let them attack you a few times. If you have the patience and time to do this, it's a fun way, at least for your first vampire toon. Getting bitten by another player is fun too, it has kind of a graphic animation, and it's much easier.