US rental companies are pretty strict about requiring a credit card to "rent" a car, although most will accept various forms of payment at the end of the rental.
Drivers under 25 tend to have this problem as well. There is a web site:
www.bnm.com that has a section called College Age Rentals. There are 4 companies listed under "Boston". I'd contact each one, explain your situation and see if they can help.
Good Luck and enjoy Boston!
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Michael Steinberg
Editor
BizTrip
http://www.biztrip.com