There is an Alamo car rental in Downtown Disney. We hired a car on our last trip from LAX and returned it in Anaheim. If I remember correctly we parked it in a parking lot near PPH and dropped the keys into the office in DTD. We plan on doing the same thing in September this year
There's a National/Alamo place down the street from Anabella Hotel on Katella. They have a shuttle and will drive you to your hotel after you drop the car off, which was unexpected but VERY cool!
Hertz and Avis both have locations right on Katella and will drop you off at the park or back at your hotel. One thing to keep in mind is that both of those locations have very limited hours -- ie. they close at 3 or 4 pm, so you have to drop the car off before that, no late night hours like an airport rental location has. I've used Avis for many years in a row for a one-way rental. This year I'm trying Hertz because their price was a bit better.