I believe it's a T-55 (or at least, they work). Isn't it the same size bolt that holds down the seats? I think it is. I just had my seats out and a T-55 worked.
In a Sears Hardware nearby, they had torx bits for a 3/8 ratchet in two different areas. The expensive ones were with the regular tools. The $5 bits were over with the automotive stuff. I bought the cheap ones.
The expensive ones were two-piece with a chromed base like a standard socket. The cheap ones were one piece with a black oxide finish.