I did some price checking for dates through August and September, through the Dollar website as well as booking sites (Travelocity and Orbitz). I could find no minivan with unlimited mileage either.

I contacted a representative from the Dollar reservation line and was told that

"at this time, we have mileage restrictions on all mini van rentals."

I think that because of the economic turndown, car rental companies are making changes in order to compensate for the loss of revenue that comes with reduced travel by consumers across the country. These changes have included closing locations, laying off employees, holding vehicles longer, and, the hardest for the customers, price increases. Car rental companies are also keeping their vehicles longer than they used to. This is due to the change in their fleet-acquisition policies; how many vehicles they buy and what kind of vehicles they buy.

In the past, car rental companies bought most of their vehicles under repurchase programs that allowed them to sell the cars, vans and SUVs back to the auto manufacturers within a certain time period. Because these programs are no longer available the rental car companies have not bought as many vehicles (especially large vehicles... luxury cars, minivans,large SUVs, pickup trucks, etc) and must keep them longer, in order to be able to get a good value when they do resell them in the used car market.

Of course, none of this helps your situation or your search to find a minivan with unlimited mileage!

I've been finding less and less minivans and large SUVs available in many locations this summer. I did some checking for various dates throughout August, September and October. I found that only Alamo, Enterprise and Hertz consistently provided unlimited mileage on their mini van rentals. I don't know if renting from another company is a possibility, but it may be something that you might want to look into.