You would go to Guest Relations in any of the theme parks to request DAS (Disability Access Service).
Once DAS is issued to a guest, it is valid in any if the 4 WDW theme parks for your entire stay (up to 60 days).
Guests can try at Guest Relations outside the park, but there is no a guarantee that it will be available there.
For all parks, if the outside of the park guest relations is busy with guests with ticket issues or they don't have the iPad they need to issue DAS, you may be sent to the inside the park Guest Relations.
It's best to be prepared in case you can't get it outside
When we returned to WDW in March 2015 DD showed the CM her previous, expired, DAS and asked if she could please have another one. They asked her if the reasons she had it for before were still current and she said yes. They did a new card and then swipped my and OH's Magic Bands so that whenever DD went onto a ride with us we could all swipe at the same time so the system knew that we all shared the return time. This way of doing it also means that we couldn't get a return time on her DAS and try and go in without her.
But if you've had a pass previously it really takes 10 minutes to get a new one. No time at all.