We tried to contact AKL about reserving a chair before leaving, but Disney wouldn't cooperate & put me through directly & I didn't bother to investigate to get a direct number. The official statement is that you can't reserve them - first come, first served. However, upon arrival, the chair I ordered from CARE couldn't be located & GS told me that I should just have called them since theirs is free. I don't know if you can do this, or they were just too new to know you can't - but it's worth a try.
It is much more convenient to have the chair available from the resort. By the time you walk from the room to the bus & then from the bus to the gate & then to w/c rental . . . It can be too much! We tried that last time & by the end of the 2nd day the CMs took pity on me & walked DH out to the WL boat launch, took the chair back for him & told him to see GS about getting one at the resort! What a relief!
AKL also had a larger chair which surprised me. I believe the parks do also if you know to ask. Last Aug when I first hurt my knee & really needed one, no one volunteered this info.
The prices I got from the outside companies were about $75 - 90 for 10 days for the larger chair. (~15 more than standard sized) They all tried to talk me into the ECV, but that would have been more like $400 for the 10 days. DH doesn't mind pushing, so I didn't want to spend the extra.
Also, if flying, be sure to ask at either end for assistance. We flew from Philly & DS kidded about walking back to Philly when going from the plane to get our luggage in Orlando!