We had to get our tickets this past August from the base that my son was assigned to. Got four 4-day park hopper tickets for $792. Called WDW and was told to go to his travel service center on his base to get the tickets:
https://disneyworld.disney.go.com/special-offers/military-multi-day-tickets/
You do have to buy them in advance in order to pre schedule fastpasses, but not all bases sell them. Mine here in CA doesn't stock them, they have to order them. For my last two trips this past year, I called Shades of Green and ordered them via phone and they mailed them to me because Shades mails actual tickets, not vouchers that need to be activated. Saves time.
Regardless, they are definitely available for purchase at the ticket booths at all the parks. In Florida, Disney charges sales tax on them (bases and Shades of Green do not). In California, at Disneyland, no tax is added because there is no sales tax on Disneyland's tickets, so I buy them at the ticket booths in CA for visiting friends and family all the time.
