The Tourist Information centres are numerous and excellent on the Isle of Wight. They have a computerised booking system for B & B's. This helps to find the right place for you. Yes they charge a commission but it's worth it just for that. If you not staying Bed and Breakfast they can still help to find camp sites etc. website: www.islandbreaks.co.uk
I use the Rambler's Yearbook and Accommodation Guide quite often for making bed and breakfast bookings. The entries in the guide are all from recommendations. They like walker's and most can cater for all your needs. The guide is issued once a year by the Ramblers Association. website: www.ramblers.org.uk
Don't chance to luck and hope you will just find somewhere late in the day. The Isle of Wight becomes busy during the usual school holiday periods and summer weekends. Yachting events can soak up all the accommodation in towns fronting the Solent. Be warned book early and turn up. If something goes wrong ring as early as possible to let your accommodation know.