You might try Simi Cycling Center 805-522-0565 or All Pro Bikes 805-583-4296 in Simi Valley. It might be difficult to locate a shop that rents road bikes but I think they do rent mountain bikes at both of these shops.

Another option would be to ask if they have a demo road bike that you could rent for the 5 days you are in town. This might cost more than one of their actual rentals but if you want a road bike it might be a option for you.

Hope you enjoy your 5 day visit to Ventura County.