OK, here is all the information I could get on rental car locations in the
Disneyland Resort Area.
Along Katella at Clementine (one block East of the Resort) there are three rental locations. On the south-side of the stree is
Avis, and on the north are Hertz and Enterprise. Further down Katella, accross from the Anaheim Convention Center is a building shared by Alamo and National. Lastly on Harbor there is a Budget (though I don't remember the cross-street).
Avis, Alamo/National, Budget, Enterprise, and Hertz provide transportation to/from hotels, and Enterprise will drop-off and pick-up the car from the hotel (this may not be true for off-property hotels such as the Carousel). As far as walking distance goes, it would be a two-block walk to any of the locations on Katella, just walk south on Harbor and then left or right based off of which location you use.
Local numbers for the offices are:
Alamo/National- 774-6250
Avis- 780-1024
Budget- 703-0281
Enterprise- 991-7195
Hertz- 772-0425
Hope this helps!