Similar to the above answer, place ads in the newspaper and contact your local shelters and vets and tell them you have an FeLV+ cat which you are willing to give to a good home. A good home is someone with another FeLV+ cat, or someone who does not have any other cats and will keep the cat indoors. Also, there are animal shelters which specifically take in FeLV+ cats. Again, contact your local vets and shelters to see if they are aware of any such haven to which you could send your cat. Almost all regular shelters will put to sleep any cat they receive which tests positive because the virus is so contagious. If you are going to do this, however, you should understand that is it unlikely that you will get instant results. If you expect to find a new home for an FeLV+ cat within a few days of making inquiries, you can pretty much forget it. Be prepared to hold on to the cat for a few weeks at least, while searching for a new home.
If you choose to put an ad in the paper you MUST take the responsibility of making sure the people who express interest in your cat are doing so for legitimate reasons. You should interview them in person and check references if possible. There are people who will lie to get cats to feed to other pets, to sell to labs or to abuse. Think about this as you interview each potential candidate.