http://archive.salon.com/travel/advisor/2000/0 1/27/mardigras/index.html
Says:
Paris isn't the best city in the world for getting around by scooter or wheelchair, what with its cobblestone and older features. But you can find help through several books and agencies.
One company with much experience arranging trips in Paris, including for travelers who need scooters, is Accessible Journeys, 35 W. Sellers Ave., Ridley Park, PA 19078; phone (800) 846-4573. Accessible Journeys has a Paris page, complete with photos of travelers using scooters. The company has a contact there who occasionally has a scooter available; it's a matter of timing.
A guidebook that goes straight to the heart of your question is "Access in Paris: A Guide for Those Who Have Problems Getting Around," by Gordon Couch and Ben Roberts (Cimino Publishing Group, 1994). It hasn't been updated, but might still be helpful to you. Excerpts are available at the Disabilitynet site. Visit Paris Tourisme's Paris A to Z pages and search for "disabled." There you'll find several phone numbers for outfits that help mobility-impaired visitors get around.
Among other sites to check are the Access-Able Travel Source and the Travelin' Talk Network, which provides information on assistance to travelers with disabilities. You may well locate someone with first-hand experience through one of these groups.
A key organization is the Society for the Advancement of Travel for the Handicapped, and its site offers many links, including a section on tour operators and agencies than can help you make plans.