I searched our MLS and there are no rentals in Niceville that are shorter than 6 months that I could find. Occasionally you might find one in the Florida Club, but it is rare. I would suggest checking out the Value Place extended stay in FWB (valueplace.com) or Resort Quest in Destin (resortquestdestin.com). Another option would be to check out Vacation Rentals By Owner (VRBO) to see if there is something in Destin or FWB/Okaloosa Island. You might also want to check out some of the local hotels to see if they have any extended stay prices. Finally, Craig's list might have some listings.
Good luck with your search.