It's always a good idea to get a second opinion from another vet.
If you seek a third or fourth opinion, you might try calling an animal hospital for information and advice on your pet's condition. Animal hospitals will often answer your questions about pet health for free.
I once had an issue with my dog in the middle of the night. She had gone completely deaf and I didn't know where to turn. I went online and looked up local 24 hour animal hospitals, found a few, and called each of them. I explained the situation and they each advised me on what to do. When I wanted a second opinion, I called another 24 hour animal hospital. Needless to say, their professional advice was a great comfort to me!
The people who work in these hospitals are there to help. They love animals and like nothing more than to give you advice and information about your pet's health and well-being. It's free, so next time you have a pet question, why not try contacting your local animal hospital?
Now for non-emergencies, websites are your best bet. There are plenty of links which are worth checking out. Just make sure that the website you visit is reliable so that you don't get inaccurate information about your pet's health. When in doubt, verify any advice that you get by comparing it to other websites!