I need to find rental car agencies that will accept debit cards
(mastercard logo) -- as opposed to credit cards.

I need one in Philadephia PA (prefably near the 30th Street Amtrak station).
And one in Austin TX (preferably at the airport, but anywhere will do).

Thanks. Having them take a deposit would be fine.

This might be useful information:

"Alamo accepts debit cards with the Visa or Mastercard logo and a
Round trip travel ticket, but restricts certain locations. Dollar
requires an acceptable credit check with debit cards. At a few limited
locations, Thrifty and some local Enterprise offices may accept debit
cards. These policies may change at any time. There are a few small
independents that allow cash deposits. With the use of a debt card,
the agency may require a credit check."

http://www.worldrentalcars.com/austin_car_rentals.html

I've called all those already and no go, and I think I need one of
those independents -- but don't have time to call through all of the
ones listed.

I am not a researcher so I can't take your money. :( Google does not
let anybody to become a researcher anymore. Anyway. Here is some help
for you for free :

I could find a close place to 30th st. train station. "Budget Rent a
Car" at 1125 NORTH BROAD STREET Landsdale,PA 19446 which accepts debit
cards at the time of rental and as a form of payment at the end of
rental. This place is appx. 33 miles from the station. There is also a
Budget "in" the train station itself but it only accepts debit cards
at the END of your rental as a payment, , meaning ; you can't rent the
car with your debit car but you can pay with it when you bring the car
back. But Landsdale location accepts in both occasions.
Budget has an office also in AUSTIN,TX Airport. This place accepts
debit cads without restrictions, as well. Please confirm these with
Budget if you want to go ahead.
  US Reservations  800 527-0700 
  Customer Service 800 214-6094 
  You can also make an online reservation at www.budget.com

  If I was getting paid I would have confirmed these with Budget
myself and bring it to you at no-hassle.But. Also I could possibly
find a closer company to the train station too. Sorry but I hope my
information helps you.
  Regards.