Go and walk the docks with credit card in hand. Always deals to be done for last minute charters in low season.

Some people, such as TMM, have their availability online http://sailtmm.com/Availability/ShowMonthBookings.cfm?B=T&SM=7&SY=2015 Of course big outfits like Moorings/Sunsail are more likely to have availability.

Good luck.