Where can I rent a Mustang Convertible?

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I am a UK-based PH-er hoping to spend 3 - 4 weeks next summer on a driving holiday in the States.

Ideally I would like to hire a Mustang convertible, or similar at one west coast airport and then drop it off on the east coast 3 weeks and 6000 miles later.

I have tried an internet search with limited success - can PH-ers on the other side of the pond help?

It doesn't need to be a Stang, but a good driver's car (ideally a manual box) is essential, convertible is desirable, no need for 4 seats.