We used Loving Donation in Florida. At first, they didn't want to reveal the donor's ID, but contacted her at our insistence and she agreed to be a "known" donor. In fact, her name is on our contract, and hubby took the twins to meet her when they were about a year old. (I had the option too but chickened out.)
Family Creations is another agency that identifies known donors.
If you want to search for a donor yourself (free ads), try the classified ads at Surromomsonline. That's how we found our first donor. Be careful though, there are some scam artists, though our first donor was wonderful and we are still in contact.
Another source of free advertising is the Brown Daily Herald (Brown University) newsletter. Although we didn't choose anyone, the girls seemed lovely and sincere.
If you find a anonymous agency donor that you really like, insist that the agency contact her to ask if she is willing to be "known." You might be pleasantly surprised.