If you want to dive with Whale Sharks you need to get to Gladden Spit
only and your best odds are right before or after the full moons in Spring/Early Summer when the Cubera Snapper Spawn which brings them in.
It's also only 4 times in the late spring/summer that it's even realistically possible you'll see one.
Some friends did it a couple years ago and dove with 8 over two trips. And it is a long ride from anywhere. They did it with Splash Dive in Placencia relatively inexpensively - the shop set them up with an inexpensive apt. in "town" and shuttled them around - including pickup at the airport.
I don't think the Turneffe resorts even go there - Turneffe Island Resort doesn't mention it at all. Neither does Turneffe Flats - they both go the other way and dive the Blue Hole and Lighthouse Reef on special trips.
The Hopkins resorts have reservations/optional trips but they're pricey. Hamanasi is expensive, Roberts Grove maybe slightly less. Turtle Inn is the highest. I believe - but do not know - that the boat ride from there would be even farther since it's north of Placencia. I guess it partly depends on the speed of the boat.
The liveaboards go nowhere near Gladden Spit unless Splash runs their
one special trip again this year.
Start Here:
Gladden Spit | Whale Sharks | Splash Dive Center | Placencia Belize or contact
Ralph Capeling - it's his business. Ralph also posts a yearly thread about it here. And it's also very date specific - the current dates are probably on their website under the Gladden Spit link. Partly explains the demand also - my friends booked in early fall for the May dates IIRC.
Avalon Divers closed a couple years ago so afaik about the only other option is
Belize Scuba Diving and Snorkeling In Placenica | Seahorse Dive Shop
fwiw, I personally have never been there although I partly planned and was supposed to go with my friends.