Looked at Alamo for a friend a couple days ago (decided to go a different direction) and they had cars for the last two weeks of July. Not only was that 2 days ago, but there's an increasing (well, there were, back in August) number of people showing up here in Los Anchorage only to be told that "We know you had a reservation but we don't have cars." ....and they are indeed expensive, circa $2000 for a week or so if one gets any add-ons.
As mentioned, Alaska 4x4 Rentals, or some of the secondary rental outlets (Rent-a-Wreck, dealer rentals, T**o, etc) might be an option as well.
Personal opinion (volunteered, worth what's paid for it) is that if one is coming to Alaska and will be on the road system (Los Anchorage-Homer-Valdez-Squarebanks-Dalton Highway in a pinch), having a vehicle exponentially improves the enjoyment of the trip. Cruise to SE, no real need for a vehicle: 3 weeks to spend between Los Anchorage and Squarebanks, having a vehicle really adds to the experience.
