They are amazing animals, huge but gentle plankton feeders. This one (above) was a juvenile at about 2.5 metres wingspan.
The best area to see them is Indonesia, especially in the Komodo Marine Park, between Flores and Sumbawa.
There is a resident population of juveniles and younger adults all year round but also the big adults come to the area to feed and breed and the best place to see them close up is one of the many "cleaning stations" on the reefs. This is where the Mantas hover to allow a myriad of smaller fish to pick off their annoying parasites and clean any wounds.
I have a
lot of photos and u/w video of Mantas from my last trip there earlier this year.
edit: Incidentally, Indonesia has recently declared them a protected species and outlawed their capture in all their waters.