This article is from the Feline Leukemia (Cats) FAQ, posted to rec.pets.cats newsgroup. Maintained by Erin Miller with numerous contributions by others.
Yes, there are several commercial vaccines available. Unfortunately the USDA does not have standard requirements for FeLV vaccines, so different manufacturers can publish 'Effectiveness Ratings' which cannot be compared with each other due to a lack of uniformity in testing terms and requirements. Estimating the effectiveness of the vaccines is difficult and most vets I've spoken with estimate them to have ~75-85% effectiveness (this means cats challenged with the virus will fight off infection 7-8 times out of ten).
Published studies show that there are irreconcilable differences and bias in Effectiveness Ratings put out by the manufacturers so currently it is impossible to estimate the actual effectiveness of and between vaccines. Suffice it to say that your cat is MUCH better off trying to fight the virus having received the vaccine, but the vaccine is not 100% effective so you should not expose your cat to undue risks. (Note: NO vaccine, whether it be for cats, dogs or people, is 100% effective. Some are about close to being as fully effective as possible, however the FeLV vaccine is not one of them).
Since writing the first version of this FAQ, I've had a number of people tell me that a cat has only a 30% likelihood of catching the disease, and even then if only in persistent close contact. I have found nothing in the recent literature to support this claim, and every veterinarian I have spoken with states that the likelihood of becoming infected depends on the level of exposure, which vary in different locales and situations. Since most tests are done in controlled situations, it would be impossible to come up with true "real world" statistics. In one vet's words "But what if it is only 30%? Would you stop wearing seat belts if there was only a 30% chance of serious accidents if you commuted in a car every day? I hope the odds are far less than that ... and yet I still hope you are taking the appropriate safety cautions. Unless the vaccine has been proven to cause serious harm, it is a good precaution to take, regardless of the percent chance of disease transmission."