I would recommend using Suntours as they have vehicles perfectly suited for your party's size and have always been professional and efficient in my dealings with them.
Agree with the above, Suntours is a great option. Alternatively, contact the hotel directly - they may have a company they frequently use and should be able to arrange it for you.
We used a guy called Nick whose company is called 'sweet fuh days' he was brilliant and made our holiday.
You will need two mini buses for 13 plus luggage
IMO ...You do need to pre book ...by Email
Good list at top LH corner of the forum page