For the months July and August, Idul Fitri period and around Christmas/New Year it is wise to book ahead. Book for a hotel which can be canceled without a fee.
Booking.com, Agoda.com and Traveloka.com offer the most choice. You also can directly contact your hotel by email or telephone.
Note that not all hotels can be booked via internet sites.
On Flores you will find few hotels of an international standard. Most of the 3 star and up hotels are in Labuan Bajo and surrounding.