I have used
Tropiclean Neem Oil Shampoo with very good results. It was at Petco or Petsmart. You have to leave it on for a good five minutes. It looks like this......
Amazon.com: Neem Shampoo - 20 oz: Kitchen &...
Petco has one by
Only Natural the is also good......
Only Natural Pet Herbal Defense Shampoo for Pets at PETCO
I ran out of my neem oil shampoo so I did a little experiment with regular pet shampoo. I left it on for 6 minutes and it killed the fleas...A couple of them where semi-conscious

so I increased the time to 7 minutes the next time and that did the trick ......After trying that a few times I now know that in a pinch I can use regular shampoo and just leave it on longer and it will kill fleas......