Here's a list of the rental places at the parks:
Magic Kingdom
-Mickey's Gift Station at the Ticket and Transportation Center (limited quantities available)
-Wheelchair and Stroller shop located to the right of the turnstiles upon entering the park
Replacement Locations:
-Tomorrowland Arcade Tinker Bell's
-Treasure in Fantasyland
- Frontier Trading Post in Frontierland
Epcot
-Gift shop just outside the Main Entrance
-Stroller and Wheelchair rental location inside the Main Entrance (
ECV's available)
-International Gateway (Wheelchairs and ECV's)
ReplacementLocations: -Glas und Porzellan in Germany
Disney MGM-Studio
-Oscar's Super Service to the right inside the Main Entrance (ECV's available)
Replacement Locations: -Tatooine Traders at Star Tours -The Writers Stop
Animal Kingdom
-Garden Gate Gifts inside the Main Entrance (ECV's available)
-Outpost (limited quantities)
Replacement Locations:
-Creature Comforts
-Mombasa Marketplace
The only park where you can rent an ecv outside the park is Epcot. The locations in the other parks are close to the entrance.
If you want to rent from one of the off-site places, you will find information in the disABILITIES FAQs thread.