Hi Armando, and welcome to the Disney Parks Moms Panel! I always love the chance to chat with a fellow Florida resident!
You can absolutely buy your Florida Resident Discount Tickets when you arrive at Walt Disney World. Just visit any of the Guest Relations windows at the entrances to the Theme Parks and provide proof of Florida residency. Unfortunately, you won't be able to use your Florida resident discount to purchase tickets for your in-laws, since all Florida Resident Discount Tickets require proof of residency for each Guest planning to use them.
One other important thing to know: there's no Florida resident discount on 1-Day Theme Park Tickets. Discounts only apply to 3- or 4-Day Florida Resident Tickets, and Florida Resident Seasonal or Annual Passes.
Also, you still have time to buy your tickets ahead of time and take advantage of Disney's FastPass+ Service to schedule times you'd like to experience your favorite attractions in advance. Just order your tickets online, link them to a My Disney Experience account, then start making FastPass+ selections right away. Then, stop by Guest Relations to pick up your tickets and show proof of residency, and let the magic begin! Here's some more information on how FastPass+ Service works. If you still have questions, please stop by and ask me another question. I'd love to help your family have a wonderful Walt Disney World visit!
I hope this helps in your planning, Armando. Have a magical day!
Kathy K.