Your timing is slightly off for sites near the U.S.
We snorkeled with six in Utila the last week of Mar, 2009. Over two days. Then it turned flat calm and drove them deep, we didn't see any more. And spent several hours per day on the boat looking. We did the Research Week with Deep Blue Utila. It's my personal feeling that they then move down to Gladden Spit in Belize for the Cubera spawn where you can dive with them in later spring/early summer. My friends did a couple years ago and
dove with 8 over two trips. Splash Belize has details on their website - or many of the southern Belize dive resort (Hamanasi etc) operators do also. Since they know the Cubera spawn before after certain full moons, they've got the dates down pretty acurately.
Then it appears they congregate in Isla Mujeres or Isla Holbox off Mexico - by the hundreds at times. Snorkel only.
I've heard they're occasionally spotted at the Flower Gardens in the Gulf of Mexico as well. But that may only be solitary encounters. Not sure when but Fling might have some details on their website.
Another possibility I know of in winter/spring is Socorros (Revililigigados Islands) in the south Sea of Cortez - all liveaboard diving and they're not always spotted there. You will see a lot of Sharks, Mantas and other large pelagics though. Those trips are likely sold out thru 2015 now but you could check with Solmar V or Rocio Del Mar - they may have space. Nautilus Explorer also runs down there still afaik. One way I figured out when it was likely was to look back at some of the Solmar V trip reports - they video one each week and post them on YouTube. IDK if you heard about the recent removal of the fishing net from one of them but that was done by Solmar's DM at Socorro. That trip is on our bucket list - maybe 2015?
Other places are Donsol in the Phillipines, Ningaloo Reef in Western Australia and the Maldives. I don't have any other info - an acquaintance is a genetics researcher studying them and she's been to those places to dive/snorkel with them. I think in our winter months but I'm not sure.
This is Australian based but you might get some ideas here:
Distribution and migration: Whale shark (Rhincodon typus) Issues Paper
Obviously the most reliable encounter is diving the big tank at the Georgia Aquarium. But that kind of takes the fun out of it.